PG18 - Remove REINDEX SYSTEM test because of aio debug lines (#8305)

Fixes #8268 

PG18 added core async I/O infrastructure. Relevant PG commit:
https://github.com/postgres/postgres/commit/da72269

In `pg16.sql test`, we tested `REINDEX SYSTEM` command, which would
later cause io debug lines in a command of `multi_insert_select.sql`
test, that runs _after_ `pg16.sql` test.

"This is expected because the I/O subsystem is repopulating its internal
callback pool after heavy catalog I/O churn caused by REINDEX SYSTEM." -
chatgpt (seems right 😄)

This PR removes `REINDEX SYSTEM` test as its not crucial anyway.
`REINDEX DATABASE` is sufficient for the purpose presented in pg16.sql
test.

src/test/regress/expected/pg16.out

@@ -878,6 +878,7 @@ SET search_path TO pg16;
 -- We already don't propagate these commands automatically
 -- Testing here with run_command_on_workers
 -- Relevant PG commit: https://github.com/postgres/postgres/commit/2cbc3c1
+-- Testing only DATABASE here as SYSTEM produces aio output in PG18
 REINDEX DATABASE;
 SELECT result FROM run_command_on_workers
 ($$REINDEX DATABASE$$);
@@ -887,15 +888,6 @@ SELECT result FROM run_command_on_workers
  REINDEX
 (2 rows)
 
-REINDEX SYSTEM;
-SELECT result FROM run_command_on_workers
-($$REINDEX SYSTEM$$);
- result
----------------------------------------------------------------------
- REINDEX
- REINDEX
-(2 rows)
-
 --
 -- random_normal() to provide normally-distributed random numbers
 -- adding here the same tests as the ones with random() in aggregate_support.sql

src/test/regress/sql/pg16.sql

@@ -521,15 +521,12 @@ SET search_path TO pg16;
 -- We already don't propagate these commands automatically
 -- Testing here with run_command_on_workers
 -- Relevant PG commit: https://github.com/postgres/postgres/commit/2cbc3c1
+-- Testing only DATABASE here as SYSTEM produces aio output in PG18
 
 REINDEX DATABASE;
 SELECT result FROM run_command_on_workers
 ($$REINDEX DATABASE$$);
 
-REINDEX SYSTEM;
-SELECT result FROM run_command_on_workers
-($$REINDEX SYSTEM$$);
-
 --
 -- random_normal() to provide normally-distributed random numbers
 -- adding here the same tests as the ones with random() in aggregate_support.sql