Remove expensive test of postgres_fdw batch inserts

Remove expensive test of postgres_fdw batch inserts

The test inserted 70k rows into a foreign table, in order to verify
correct behavior with more than 65535 parameters, and was added in
response to a bug report.

However, this is rather expensive, especially when running the tests
under valgrind, CLOBBER_CACHE_ALWAYS etc. It doesn't seem worth it to
keep running the test, so remove it from all branches (14+).

Backpatch-through: 14
Discussion: https://postgr.es/m/2131017.1623451468@sss.pgh.pa.us