drm/amd/display: Remove unused variable

Remove an unused variable "remove_disconnect_edp" which was a workaround
bit.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>