res_mwi_external: Clear the stasis cache entry when the external MWI is deleted.

One of the things missing when external MWI support was added was the
ability to clear the stasis cache entry of deleted external MWI mailboxes.

Review: https://reviewboard.asterisk.org/r/3325/
........

Merged revisions 410555 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410557 65c4cc65-6c06-0410-ace0-fbb531ad65f3