Telephony: don't show unsupported transfer on IMS

Ims phone calls do not have an impl to support call transfers yet; do
not report that we support it.

Ticket: CYNGNOS-2760

Change-Id: I43e97c7d98d239e68204e0d58cafd82f38dd10e3
Signed-off-by: Roman Birg <roman@cyngn.com>