IB/rdmavt: Remove signal_supported and comments

Initially it was intended that rdmavt would support some signaling
between the underlying driver and itself. However this turned out to be
unnecessary for qib and hfi1. If we need to add something like this in
later to support another driver we should do it then. As of now this
essentially dead code so remove it.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>