Don't use pgbench -j in tests

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

Added by commit 038f586d5f, 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

src/bin/pgbench/t/001_pgbench_with_server.pl

@@ -878,7 +878,7 @@ select 1 \gset f
 
 # Working \startpipeline in prepared query mode with serializable
 $node->pgbench(
-	'-c4 -j2 -t 10 -n -M prepared',
+	'-c4 -t 10 -n -M prepared',
 	0,
 	[
 		qr{type: .*/001_pgbench_pipeline_serializable},