Remove unused core scmi_xfer wrappers now that we have migrated all
protocols to the new interface based on protocol handles.

Link: https://lore.kernel.org/r/20210316124903.35011-34-cristian.marussi@arm.comTested-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>