tests: Remove useless run-if-fail commands passed to AT_CHECK

tests: Remove useless run-if-fail commands passed to AT_CHECK

Path to ovsdb-server's pidfile has changed long ago when
ovsdb_start_idltest() helper was introduced in commit 561205007e17
("tests: Get rid of overly specific --pidfile and --unixctl options.")
but the run-if-fail commands were left behind.

Besides, we don't need to kill the ovs-db-server from the AT_CHECK
anymore since ovsdb_start_idltest() registers an on_exit hook that will
do it.

Clean up any run-if-fail commands that attempt to kill ovsdb-server
using an invalid pidfile.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>