Fix the default value for DeferShardDeleteOnMove

The default for GUC citus.defer_drop_after_shard_move is true. However
we initialize the global variable with a false value.

src/backend/distributed/operations/repair_shards.c

@@ -112,7 +112,7 @@ PG_FUNCTION_INFO_V1(master_copy_shard_placement);
 PG_FUNCTION_INFO_V1(citus_move_shard_placement);
 PG_FUNCTION_INFO_V1(master_move_shard_placement);
 
-bool DeferShardDeleteOnMove = false;
+bool DeferShardDeleteOnMove = true;
 
 double DesiredPercentFreeAfterMove = 10;
 bool CheckAvailableSpaceBeforeMove = true;