drm/amd/display: Delete unused dce_clk_mgr.c

drm/amd/display: Delete unused dce_clk_mgr.c

This file was not compiled or included anywhere and not
modified for years. And it has duplicate function
definitions of many functions that are defined in
other files.

That leads to a lot of confusion (for both developers
and for code editors), eg. when searching for a definition
of a function it can end up on one of the functions in
this file instead of the real implementations.

I suspect that it was forgotten that this should be
deleted after the various clock manager functions were
split into hardware generation specific files.
Delete it now.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>