Remove atomics from g_clear_object/g_clear_pointer

Practically no caller of these functions require atomic behaviour,
but the atomics are much slower than normal operations, which makes
it desirable to get rid of them. We have not done this before because
that would be a break of the ABI.

However, I recently looked into this and it seems that even if the
atomics *are* used for g_clear_* it is not ever safe to use this.  The
atomics protects two threads that are racing to free a global/shared
object from freeing the object twice. However, any *user* of the global
object have no protection from the object being freed while in use,
because there is no paired operation the reads and refs the object
as an atomic unit (nor can such an operation be implemented using
purely atomic ops).

So, since nothing could safely have used the atomic aspects of these
functions I consider it acceptable to just remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=733969