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
Merge pull request #1339 from Wikia/AfterLanguageGetNamespaces-WallFix

Improve performance of Wall code