drm/amd/display: remove unused header

dml_wrapper* files were removed in commit 01b537ee
("drm/amd/display: Remove unused code"), as they are not used anywhere.
However, the header file wasn't removed, so remove the header as well.

Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>