The tunneldigger init script has been migrated to procd, so there is no
PID file anymore. Remove the obsolete check altogether - it shouldn't
be needed anymore, as procd will take care of crashes of tunneldigger by
itself. This fixes the watchdog resetting tunneldigger every time it runs.

Closes #2977

This allows the script to be minified as well as being checked by
luacheck.

The logic is based on the previously used shell script implementation.

Co-Authored-By: Robin Weiligmann <admin@robwei.me>