Revert "gatomic: statically assert that our assumptions hold"

This reverts commit 7269d75321b9d2a967a59fb35f243397f577eb41.

Adding G_STATIC_ASSERT() into a header file caused compilation
problems with at least one app (Anjuta). Reverting to keep
GNOME continuous testing running.

https://bugzilla.gnome.org/show_bug.cgi?id=730932