Temporarily(?) remove BRIN isolation test.

Commit 2834855 added a not-very-carefully-thought-out isolation test
to check a BRIN index bug fix.  The test depended on the availability
of the pageinspect contrib module, which meant it did not work in
several common testing scenarios such as "make check-world".  It's not
clear whether we want a core test depending on a contrib module like
that, but in any case, failing to deal with the possibility that the
module isn't present in the installation-under-test is not acceptable.

Remove that test pending some better solution.

Author: tglsfdc

src/test/isolation/expected/brin-1.out

@@ -36,7 +36,6 @@ test: skip-locked
 test: skip-locked-2
 test: skip-locked-3
 test: skip-locked-4
-test: brin-1
 test: drop-index-concurrently-1
 test: alter-table-1
 test: alter-table-2