drm/radeon: drop btc_get_max_clock_from_voltage_dependency_table

It's no longer used now that the underlying bugs are fixed.

Reviewed-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Tested-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>