mark_overlays: Use the normal ITREE_FOREACH

mark_overlays: Use the normal ITREE_FOREACH

This commit basically reverts commit 5b954f8f9.  The problem of nested
iterations hasn't been fixed in the mean time, but since the GC can
run arbitrary ELisp code (via `post-gc-hook`), running the GC from
within an itree iteration is already unsafe anyway :-(

* src/alloc.c (mark_overlays): Delete function.
(mark_buffer): Use ITREE_FOREACH.