drm/amd/display: removed unused function dc_link_reallocate_mst_payload.

[Why]
Found that dc_link_reallocate_mst_payload is not used anymore
in any of the use case scenario.

[How]
removed dc_link_reallocate_mst_payload function definition
and declaration.

Signed-off-by: Robin Singh <robin.singh@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>