Improve performance of Wall code

This change reduces calls to WallHooksHelper::onLanguageGetNamespaces to just one. Used to be 200+ on pages with looooots of links.

* partially revert e5458fbb088cb
* remove AfterLanguageGetNamespaces custom hook
* use LanguageGetNamespaces hook which is called only once and then the result is cached

@themech - please review and merge