Fix vcregress.pl for new plperl test case.

As of commit 50fc694e, the plperl tests don't want preinstalled
plperl languages; they now install those themselves.  I'd removed the
--load-extension options from the GNUmakefile, but missed teaching
the MSVC build script about that.

Per buildfarm.

src/tools/msvc/vcregress.pl

@@ -353,8 +353,8 @@ sub plcheck
 		if ($lang eq 'plperl')
 		{
 
-			# run both trusted and untrusted perl tests
-			push(@lang_args, "--load-extension=plperlu");
+			# plperl tests will install the extensions themselves
+			@lang_args = ();
 
 			# assume we're using this perl to built postgres
 			# test if we can run two interpreters in one backend, and if so