Remove obsolete module development docs

As far as I can see, it's no longer possible to build the docs one-by-one, so there is no way to replace this. But `make webdocs` doesn't seem to work for me either (fails on some missing images), so maybe it's better to just mention something like this:

    $ mkdir tmp_docs
    $ ./hacking/module_formatter.py -t rst -M library/ -o tmp_docs
    $ less tmp_docs/git_module.rst

Or possibly:

    $ mkdir tmp_docs
    $ ./hacking/module_formatter.py -t rst -M library/ -o tmp_docs
    $ rst2man tmp_docs/git_module.rst > ansible-git.1
    $ man ./ansible-git.1

This builds docs for all the modules, it still does less work than `make webdocs`.