regulator: rk8xx: remove unused functions

Those two functions had their last user removed in commit f9c68a56
("rockchip: phycore_rk3288: remove phycore_init() function") part of
v2023.01 release, so let's do some cleanup here.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>