Don't enable fsync in src/test/recovery/t/008_fsm_truncation.pl.

In adverse circumstances, the fsync calls cause this test to run for
quite a long time (multiple minutes) and even suffer timeout failures.
This seems to date from before we made an effort to disable fsync in
all our test cases; there's not a lot of point in using it if there's
not a plan to force an O/S crash during the test.

Discussion: https://postgr.es/m/440239.1642560607@sss.pgh.pa.us

src/test/recovery/t/008_fsm_truncation.pl

@@ -17,7 +17,6 @@ $node_primary->init(allows_streaming => 1);
 
 $node_primary->append_conf(
 	'postgresql.conf', qq{
-fsync = on
 wal_log_hints = on
 max_prepared_transactions = 5
 autovacuum = off