Remove unstable test suite added by 525392d57

Remove unstable test suite added by 525392d57

The 'cached-plan-inval' test suite, introduced in 525392d57 under
src/test/modules/delay_execution, aimed to verify that cached plan
invalidation triggers replanning after deferred locks are taken.
However, its ExecutorStart_hook-based approach relies on lock timing
assumptions that, in retrospect, are fragile. This instability was
exposed by failures on BF animal trilobite, which builds with
CLOBBER_CACHE_ALWAYS.

One option was to dynamically disable the cache behavior that causes
the test suite to fail by setting "debug_discard_caches = 0", but it
seems better to remove the suite. The risk of future failures due to
other cache flush hazards outweighs the benefit of catching real
breakage in the backend behavior it tests.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2990641.1740117879@sss.pgh.pa.us