drm/amd/display: Remove unused struct freesync_context

All references to struct freesync_context were removed, so remove the
struct freesync_context itself and its entry on struct dc_stream_state.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>