Remove drmmode_terminate_leases

The RandR screen private is already freed when our CloseScreen runs, so
this can't do anything useful. This cleanup has to be done by the X
server itself.

(Ported from amdgpu commit 5f06d6b8ba570b500956ad26fee711d5ac427818)
Acked-by: Alex Deucher <alexander.deucher@amd.com>