journald: remove unconditional log_*() use in the main loop

Setting MaxRetentionSec= caused the kernel log to overflow and the
journal daemon to enter an endless loop.

Logging from the journald main loop gets directed to /dev/kmsg,
which wakes up journald again. We skip the import of this message
by checking for our own PID, but this still causes the main loop
to never go to sleep again because we never stopped logging from
there.