Remove incomplete code from MergePolicy module

Quite a lot of the code in Retcon.MergePolicy was related to a
designed but not yet implemented feature allowing policies to be
composed using a small suite of combinators.

Let's delete that unnecessary complexity unless and until we decide we
want to implement those features.
Merge pull request #74 from anchor/wip-polish-mergepolicies

Remove incomplete code from MergePolicy module