BannedContactModel: prevent remove() call with invalid args

The application crashes if the ContactMethod given to
BannedContactModel::remove() doesn't exist in the banned list.
Check for existance and no-op if not.

Change-Id: Ifde1c1b9a0c369889c8dabfce20dd8d8937b715b
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>