drm/amdgpu: Remove one duplicated ef removal

That has been done in BO release notify.

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>