PhoneDirectoryModel: remove incomingMessage slot

IM has nothing related to a PhoneDirectory.
IM signals are already handled by media::Text class.
Code duplication that may results to bugs.

Change-Id: Ie4d26d76d7267ab73a46f4fc5224a4b52e0466bd
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>