firmware: arm_scmi: Remove legacy transport-layer code

Since all SCMI transports have been made standalone drivers, remove all the
core SCMI stack legacy support that was needed to run transports as built
into the stack.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240812173340.3912830-10-cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>