drm/amd/display: Remove Legacy FIXED_VS Transparent LT Sequence

The New sequence has been in use in DCN314 with no regressions
introduced. Therefore, it is safe to enable this sequence for all
devices using FIXED_VS retimers. So, remove the legacy codepath and its
associated config flag.

Reviewed-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>