(Bug 41667) remove onPageContentLanguage handler.

Authored by daniel.


(Bug 41667) remove onPageContentLanguage handler.

RepoHooks::onPageContentLanguage is no longer needed, because EntityHandler
overrides getPageViewLanguage() to return the user's language, and
isParserCacheSupported() to return false. There is no need to pretend that
the page's original content language is the user's language.

If this should cause problems, it is suggested to implement whatever logic
is desired by overriding EntityHandler::getPageLanguage directly.

Change-Id: Ic8a2d3c0ab47203b3a77040d502a75e517b9f321