Cleanup for r42022/r42023 interwiki stuff

Cleanup for r42022/r42023 interwiki stuff
* Remove duplicate code in Interwiki::isValidInterwiki(), which should help avoid future drift between the two copies
* Early return on prefix = '', we know it won't be valid :)
* Dump extra isValidInterwikiCached function -- just go through the regular fetch() path, it doesn't do much more processing.
* Simplify Title codepath in full URL generation to ensure we never call an invalid object