pgsql: Don't use pgbench -j in tests

Don't use pgbench -j in tests

It draws an unnecessary error in builds compiled without thread support.

Added by commit 038f586d5f1d, which was backpatched to 14; though in
branch master we no longer support such builds, there's no reason to
have this there, so remove it in all branches since 14.

Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/ZW2G9Ix4nBKLcSSO@paquier.xyz

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/419cac053b6b27e4111216b8a529a1a0887580bb

Modified Files
--------------
src/bin/pgbench/t/001_pgbench_with_server.pl | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)