Remove EVLOOP_ONCE calls in filescanner
These calls generate error messages when the loop is already running, which
will be the case when using init-rescan or full-rescan. The only purpose
of these calls seems to be to check for exit signals from the main thread
during a startup bulk scan, where the loop is not running yet. However, we
can check for an exit signal by just setting/checking scan_exit.

This commit also removes the unused filescanner_status().