resourceloader: Remove redundant back-compat in mw.loader.addSource()

resourceloader: Remove redundant back-compat in mw.loader.addSource()

Follows-up e103ba265b.

This method doesn't need to be backward-compatible as this is already
normalised server-side.

When upgrading from before MediaWiki 1.24 the startup response will
change to the new format and also inline a request to the newer
version of mediawiki.js.

Also update method documentation to be up to date and remove
spurious return statements that aren't used.

Change-Id: I03d1b5666fcddedaa46ecf878ee7636042a5e3be