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