drm/doc: Remove todo item about "This is gross" comment

This patch remove the todo item "Use new IDR deletion interface to
clean up drm_gem_handle_delete()" after it has been resolved with
the commit "drm: Remove obsolete "This is gross" comment".
Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170926210835.GA4622@Haneen

Documentation/gpu/todo.rst

@@ -184,12 +184,6 @@ Contact: Sean Paul, Maintainer of the driver you plan to convert
 Core refactorings
 =================
 
-Use new IDR deletion interface to clean up drm_gem_handle_delete()
-------------------------------------------------------------------
-
-See the "This is gross" comment -- apparently the IDR system now can return an
-error code instead of oopsing.
-
 Clean up the DRM header mess
 ----------------------------