Remove default meta_include_file

XHTML/HTML5 don't validate the use of http-equiv meta tags to control
caching.

Browser support for these legacy meta tags is not necessarily valid:

Chrome has an open bug regarding not implementing them at
https://code.google.com/p/chromium/issues/detail?id=2763

Firefox equally may not make use of the meta tags:
https://support.mozilla.org/en-US/questions/943615

The general suggested method for setting caching controls,
such that proxies etc respect them is HTTP Headers - which we set within
Mantis at this time.