app_voicemail: Remove MessageExists and MESSAGE_EXISTS()
* The MailboxExists dialplan application was deprecated on 2006-09-26
in Asterisk 1.6.0 (commit ec83b111)
* The MAILBOX_EXISTS dialplan function was deprecated on 2011-12-06 in
Asterisk 11.0.0 (commit fd64bb66)
Change-Id: I71cfc9d7b9217a37b802f4cc6ef2d57900b7398f