simplify SfxListener::EndListening and StartListening

No-one is using the return values, so we can simplify the methods

Change-Id: Icc2dbfb797bcd82afcbf42ed3bdc20b0426ccba1
Reviewed-on: https://gerrit.libreoffice.org/9542
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>