Remove the flaky rollback_to_savepoint test (#6190)

This removes a flaky test that I introduced in #3868 after I fixed the
issue described in #3622. This test is sometimes fails randomly in CI.
The way it fails indicates that there might be some bug: A connection
breaks after rolling back to a savepoint.

I tried reproducing this issue locally, but I wasn't able to. I don't
understand what causes the failure.

Things that I tried were:

1. Running the test with:
   ```sql
   SET citus.force_max_query_parallelization = true;
   ```
2. Running the test with:
   ```sql
   SET citus.max_adaptive_executor_pool_size = 1;
   ```
3. Running the test in parallel with the same tests that it is run in
   parallel with in multi_schedule.

None of these allowed me to reproduce the issue locally.

So I think it's time to give on fixing this test and simply remove the
test. The regression that this test protects against seems very unlikely
to reappear, since in #3868 I also added a big comment about the need
for the newly added `UnclaimConnection` call. So, I think the need for
the test is quite small, and removing it will make our CI less flaky.

In case the cause of the bug ever gets found, I tracked the bug in #6189

Example of a failing CI run:
https://app.circleci.com/pipelines/github/citusdata/citus/26098/workflows/f84741d9-13b1-4ae7-9155-c21ed3466951/jobs/736424

For reference the unexpected diff is this (so both warnings and an error):
```diff
 INSERT INTO t SELECT i FROM generate_series(1, 100) i;
+WARNING:  connection to the remote node localhost:57638 failed with the following error: 
+WARNING:  
+CONTEXT:  while executing command on localhost:57638
+ERROR:  connection to the remote node localhost:57638 failed with the following error: 
 ROLLBACK;
```

This test is also mentioned as the most failing regression test in #5975

src/test/regress/expected/rollback_to_savepoint.out

@@ -1,27 +0,0 @@
--- Regression test for this issue:
--- https://github.com/citusdata/citus/issues/3622
-SET citus.shard_count = 4;
-SET citus.next_shard_id TO 1954000;
-CREATE SCHEMA rollback_to_savepoint;
-SET search_path TO rollback_to_savepoint;
-CREATE TABLE t(a int);
-SELECT create_distributed_table('t', 'a');
- create_distributed_table
----------------------------------------------------------------------
-
-(1 row)
-
--- This timeout is chosen such that the INSERT with
--- generate_series(1, 100000000) is cancelled at the right time to trigger the
--- bug
-SET statement_timeout = '2s';
-BEGIN;
-INSERT INTO t VALUES (4);
-SAVEPOINT s1;
-INSERT INTO t SELECT i FROM generate_series(1, 10000000) i;
-ERROR:  canceling statement due to statement timeout
-ROLLBACK TO SAVEPOINT s1;
-INSERT INTO t SELECT i FROM generate_series(1, 100) i;
-ROLLBACK;
-DROP SCHEMA rollback_to_savepoint CASCADE;
-NOTICE:  drop cascades to table t

src/test/regress/multi_schedule

@@ -64,7 +64,7 @@ test: tableam
 # ----------
 # Miscellaneous tests to check our query planning behavior
 # ----------
-test: multi_deparse_shard_query multi_distributed_transaction_id intermediate_results limit_intermediate_size rollback_to_savepoint
+test: multi_deparse_shard_query multi_distributed_transaction_id intermediate_results limit_intermediate_size
 test: multi_explain hyperscale_tutorial partitioned_intermediate_results distributed_intermediate_results multi_real_time_transaction
 test: multi_basic_queries cross_join multi_complex_expressions multi_subquery multi_subquery_complex_queries multi_subquery_behavioral_analytics
 test: multi_subquery_complex_reference_clause multi_subquery_window_functions multi_view multi_sql_function multi_prepare_sql

src/test/regress/sql/rollback_to_savepoint.sql

@@ -1,23 +0,0 @@
--- Regression test for this issue:
--- https://github.com/citusdata/citus/issues/3622
-SET citus.shard_count = 4;
-SET citus.next_shard_id TO 1954000;
-CREATE SCHEMA rollback_to_savepoint;
-SET search_path TO rollback_to_savepoint;
-
-CREATE TABLE t(a int);
-SELECT create_distributed_table('t', 'a');
-
--- This timeout is chosen such that the INSERT with
--- generate_series(1, 100000000) is cancelled at the right time to trigger the
--- bug
-SET statement_timeout = '2s';
-BEGIN;
-INSERT INTO t VALUES (4);
-SAVEPOINT s1;
-INSERT INTO t SELECT i FROM generate_series(1, 10000000) i;
-ROLLBACK TO SAVEPOINT s1;
-INSERT INTO t SELECT i FROM generate_series(1, 100) i;
-ROLLBACK;
-
-DROP SCHEMA rollback_to_savepoint CASCADE;