Remove race-prone hot_standby_feedback test cases in 001_stream_rep.pl.

Remove race-prone hot_standby_feedback test cases in 001_stream_rep.pl.

This script supposed that if it turned hot_standby_feedback on and then
shut down the standby server, at least one feedback message would be
guaranteed to be sent before the standby stops.  But there is no such
guarantee, if the standby's walreceiver process is slow enough --- and
we've seen multiple failures in the buildfarm showing that that does
happen in practice.  While we could rearrange the walreceiver logic to
make it less likely, it seems probably impossible to create a really
bulletproof guarantee of that sort; and if we tried, we might create
situations where the walreceiver wouldn't react in a timely manner to
shutdown commands.  It seems better instead to remove the script's
assumption that feedback will occur before shutdown.

But once we do that, these last few tests seem quite redundant with
the earlier tests in the script.  So let's just drop them altogether
and save some buildfarm cycles.

Backpatch to v10 where these tests were added.

Discussion: https://postgr.es/m/1922.1531592205@sss.pgh.pa.us