[Python] Simplify installation instructions

[bjwatson]

What:

Revise http://pythonhosted.org/gax-google-pubsub-v1/starting.html#installation as follows:

Installation
-----------

..code-block:: console

    pip install gax-google-pubsub-v1
It's recommended to use virtualenv(link).
For OS X / Linux::
    ...
For windows::
    ...

@jonparrott recommends the Request documentation as a good example.

Why:

In general, we should just show the simplest thing first (see https://googlecloudplatform.github.io/gcloud-python/stable/).

How:

This is controlled by either or both of:

• templates/gax/python/README.rst.mustache
• templates/gax/python/docs/starting.rst.mustache

Remove one of these if it's dead code.