Remove Fog.class_from_string

This was a helper for the VSphere provider that was attached to the
global Fog module.

It was not used anywhere else so does not warrant begin maintained as
part of core.

It is dealt with in fog in https://github.com/fog/fog/pull/2787
Merge pull request #29 from fog/remove_class_from_string_helper

Remove Fog.class_from_string