roster: merge roster_add_contact_cb and roster_remove_contact_cb to change_roster_iq_cb
roster: merge roster_add_contact_cb and roster_remove_contact_cb to change_roster_iq_cb

There is no point to have 2 different callbacks containing exactly the
same code. Merge them in a single callback.

freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,noonien-d/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky