Remove unstable pg_amcheck tests.
Recent pg_amcheck bugfix commit
d2bf06db added a test case that the
buildfarm has shown to be non-portable. It doesn't particularly seem
worth keeping anyway. Remove it.
Discussion: https://postgr.es/m/CAH2-Wz=7HKJ9WzAh7+M0JfwJ1yfT9qoE+KPa3P7iGToPOtGhXg@mail.gmail.com
Backpatch: 14-, just like the original commit.