Issue 4135/1: Don't use ASSERT_LIVE_IS_ALLOWED in simple smobs

Issue 4135/1: Don't use ASSERT_LIVE_IS_ALLOWED in simple smobs

When replacing mark_smob with a proper member function, use of
ASSERT_LIVE_IS_ALLOWED would require access to self_scm () or similar,
and simple smobs don't link back to their SCM cell.  This only
concerns Skyline and Context_mod, so the loss is not all that
large.