core: rework how logging level is calculated for kill operations

Setting the log level based on the signal made sense when signals that
were used were fixed. Since we allow signals to be configured, it doesn't
make sense to log at notice level about e.g. a restart or stop operation
just because the signal used is different.

This avoids messages like:
  six.service: Killing process 210356 (sleep) with signal SIGINT.