drm/amd/display: Delete dead code in command_table_helper

[Why]
dig_encoder_sel_to_atom will always return zero on any ASIC version
past DCE80 since programming of the FE selection is handled by
driver, but the translation code was left in the function, making
it look like a coding error.

[How]
Remove code that has no effect, and replace with a comment describing
why it returns zero.

Signed-off-by: Julian Parkin <julian.parkin@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>