drm/amd/dc: remove unused variable 'video_optimized_pixel_rates'

drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_clock_source.c:1017:50:
 warning: ‘video_optimized_pixel_rates’ defined but not used [-Wunused-const-variable=]
 static const struct pixel_rate_range_table_entry video_optimized_pixel_rates[] = {
                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~

commit d8cd587d ("drm/amd/display: removing MODULO change for dcn2")
left behind this unused vairable, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>