PYTHON-526 Remove the network_timeout query option.

This was always a misguided feature. Using a socket timeout
to "timeout" MongoDB queries is awful for a few reasons:

- It leaves the query running on the server, wasting resources.
- It causes a socket to be closed and discarded unnecessarily.
- Applications that use this tend to immediately retry the query,
often causing another timeout and more wasted resources.

There are three remaining ways to use timeouts with PyMongo:

- connectTimeoutMS - Global setting for timing out a socket's
initial connection to the server.
- socketTimeoutMS - Global setting for timing out any socket
operation. *Do not* use this to enforce time limits on server
side operations.
- max_time_ms (or maxTimeoutMS as a command kwarg) - MongoDB 2.6
server side timebomb for queries and commands. This is a server
side option, passed along with a query or command, that causes
MongoDB to abort the operation if it takes longer than the specified
limit.