Fix invalid-parallel-jobs error message

Including the program name twice is not helpful:

-> pg_dump -j -1
pg_dump: pg_dump: invalid number of parallel jobs

Correct by removing the progname from the exit_horribly() call used when
validating the number of parallel jobs.

Noticed while testing various pg_dump error cases.

Back-patch to 9.3 where parallel pg_dump was added.

src/bin/pg_dump/pg_dump.c

@@ -623,7 +623,7 @@ main(int argc, char **argv)
 		|| numWorkers > MAXIMUM_WAIT_OBJECTS
 #endif
 		)
-		exit_horribly(NULL, "%s: invalid number of parallel jobs\n", progname);
+		exit_horribly(NULL, "invalid number of parallel jobs\n");
 
 	/* Parallel backup only in the directory archive format so far */
 	if (archiveFormat != archDirectory && numWorkers > 1)