Remove easy_install from documentation

pip has been the preferred installer since Python 2.7:

pip is the preferred installer program. Starting with Python 2.7.9, it is included by default with the Python binary installers.

In addition to that, easy_install has been deprecated since v42 (November 2019):
https://setuptools.readthedocs.io/en/latest/history.html#v42-0-0

We should remove easy_install from pymongo's documentation.