chore(rpi3): remove redundant code

The pwr_domain_pwr_down_wfi entry is overridden by a newer
implementation. This removes the last reference to
rpi3_pwr_domain_pwr_down_wfi. Remove both as they are not needed

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ie65c40935cd1ed3c673ffdc9aa72064f5ab4032e