Fix intermittent BF failures in 035_conflicts.

This commit addresses two sources of instability in the 035_conflicts
test:

Unstable VACUUM usage:
The test previously relied on VACUUM to remove a deleted column, which can
be unreliable due to concurrent background writer or checkpoint activity
that may lock the page containing the deleted tuple. Since the test
already verifies that replication_slot.xmin has advanced sufficiently to
confirm the feature's correctness, so, the VACUUM step is removed to
improve test stability.

Timing-sensitive retention resumption check:
The test includes a check to confirm that retention of conflict-relevant
information resumes after setting max_retention_duration to 0. However, in
some cases, the apply worker resumes retention immediately after the
inactive slot is removed from synchronized_standby_slots, even before
max_retention_duration is updated. This can happen if remote changes are
applied in under 1ms, causing the test to timeout while waiting for a
later log position. To ensure consistent behavior, this commit delays the
removal of synchronized_standby_slots until after max_retention_duration
is set to 0.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/TY4PR01MB16907805DE4816E53C54708159414A@TY4PR01MB16907.jpnprd01.prod.outlook.com

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