firmware: arm_scpi: remove scpi_remove

sysfs_create_groups and of_platform_populate can be replaced with the
device-managed versions what allows us to remove scpi_remove.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>