drm/amd/display: remove unused function dc_link_perform_link_training

This function is not used anywhere, remove it. It was added in
40dd6bd3 ("drm/amd/display: Linux Set/Read link rate and lane count
through debugfs") and moved in fe798de5 ("drm/amd/display: Move link
functions from dc to dc_link"), but a user is missing.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>