Email Notification: removed priority
Email Notification: removed priority

Removed listing of priority in Email keys. No known protocol
support it at the moment and it's unclear what value should be
available.

Signed-off-by: Nicolas Dufresne <36c35e0aac84fcb59b447cc6415df42e7fda8df7@collabora.co.uk>

TelepathyIM/telepathy-spec,freedesktop-unofficial-mirror/telepathy__telepathy-spec