Remove atomicfile class.
The interface provided by opener(atomic=True) is inherently unsafe:
if an exception is raised in the code using the atomic file, the
possibly incomplete file will be renamed to its final destination,
defeating the whole purpose of atomic files.
To get around this, we would either need some bad hacks involving
sys.exc_info (to make sure things work in except: blocks), or an
interface to say "file is complete; rename it".
This is the exact interface provided by atomictempfile. Since there
are no remaining users of the atomicfile class, just remove it.