drm/amd/display: Remove unused dcn316_smu_set_voltage_via_phyclk function

Remove dcn316_smu_set_voltage_via_phyclk function, which is not used in the
codebase.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/clk_mgr/dcn316/dcn316_smu.c:171:5:
warning: no previous prototype for function
'dcn316_smu_set_voltage_via_phyclk' [-Wmissing-prototypes]
int dcn316_smu_set_voltage_via_phyclk(struct clk_mgr_internal *clk_mgr, int
requested_phyclk_khz)
    ^

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>