Undo extra chattiness of postmaster logs in TAP tests.

Commit 6271fceb8 changed PostgresNode.pm to force log_min_messages = debug1
in all TAP tests, without any discussion and without a concrete need for
it.  This makes some of the TAP tests noticeably slower (although much of
that may be due to poorly-written regexes), and for certain it's bloating
the buildfarm logs.  Revert the change.

Discussion: https://postgr.es/m/32459.1525657786@sss.pgh.pa.us

src/test/perl/PostgresNode.pm

@@ -442,7 +442,6 @@ sub init
 	print $conf "restart_after_crash = off\n";
 	print $conf "log_line_prefix = '%m [%p] %q%a '\n";
 	print $conf "log_statement = all\n";
-	print $conf "log_min_messages = debug1\n";
 	print $conf "log_replication_commands = on\n";
 	print $conf "wal_retrieve_retry_interval = '500ms'\n";
 	print $conf "port = $port\n";