firmware: arm_scmi: Cleanup events registration transient code

Remove all the events registration code used to ease the transition to the
new interface based on protocol handles.

Link: https://lore.kernel.org/r/20210316124903.35011-35-cristian.marussi@arm.com


Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>