drm/amd/display: dcn20: remove an unused function

The dpp2_get_optimal_number_of_taps function is never used. Removing
just for code cleaning up.

Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>