drm/amdgpu: Remove ppatomfwctrl deadcode

pp_atomfwctrl_get_pp_assign_pin() and pp_atomfwctrl_get_pp_assign_pin()
were added in 2017 by
commit 0d2c7569 ("drm/amdgpu: add new atomfirmware based helpers for
powerplay")
but have remained unused.

Remove them, and the helper functions they used.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>