Remove query_cache=0 documentation

based off https://bugs.launchpad.net/codership-mysql/+bug/1279220 it
seems query_cache enable is possible. Lets show this in the
documentation.
merge from 3.x
add code-block:: ini docs/source/configuration.rst
Merge pull request #68 from openquery/query_cache_possible

Remove query_cache=0 from documentation