Remove custom caching headers

According to commit summary[1], caching of responses to frontend
proxies was introduced as a stopgap. It doesn't seem needed now,
MF should rely on core for headers as much as possible.

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[1] http://svn.wikimedia.org/viewvc/mediawiki/trunk/extensions/MobileFrontend/MobileFrontend.php?revision=93405&view=markup

Change-Id: Ie7b3259fbddcbd71efd76c30dfe613d22ccca6a4
Merge "Remove custom caching headers"