impatomic: remove fence member functions

Recent C/C++ drafts do not include fence() as a member function for any
object. All fences apply globally (not to any particular object).

[ These changes came in the ISO papers N2633, N2731, N2752 ]

So, I discard any reference to a fence that is associated with an object
(i.e., as a member of an object or with an object parameter).

I will implement a replacement shortly, which just uses the
_ATOMIC_FENCE_(order) macro.