drm/amd/display: Remove mst_hotplug_work

[Why]
The work struct's schedule call was removed a while ago, making this
useless.

[How]
Remove it.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>