Remove reference to non-existent help file

(closes issue #15427)
 Reported by: brushtyler
 Patches: 
       app_voicemail.c.diff uploaded by brushtyler (license 821)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@206185 65c4cc65-6c06-0410-ace0-fbb531ad65f3