Makefile.am: remove stale intltool-merge lockfile

Sometimes it might happen that a stale lockfile is left over,
causing distcheck to fail (causing the gitlab pipeline to fail).

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
(cherry picked from commit f5235c37)
2 jobs for !47 with submit/bugfixes-4.16 in 2 minutes and 38 seconds (queued for 2 seconds)
latest merge request