net/mwan3: remove pid file use pgrep to get pid

net/mwan3: remove pid file use pgrep to get pid

If mwan3track will not stop immediately after sending the kill signal,
the clean_up handler will delete the pid file later while the new mwan3track is
already running.

This could result in a situation that mwan3track is running
more then once because the old mwan3track service could not be killed,
because the pid file is missing.

Using pgrep to kill all mwan3track for the tracked interface and not using
pid file should fix this issue.

Signed-off-by: Florian Eckert <redacted>