app_voicemail: Fix compile breaking in app_voicemail with IMAP_STORAGE.

There is a leftover "assert" in app_voicemail/__messagecount that references 
variables that don't exist.  This causes the compile to fail when 
--enable-dev-mode and IMAP_STORAGE are selected.

This patch removes the assert.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4461/
........

Merged revisions 432484 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432485 65c4cc65-6c06-0410-ace0-fbb531ad65f3