Remove mediawiki.special.javaScriptTest module

Remove mediawiki.special.javaScriptTest module

We're almost ready to drop the non-plain mode of Special:JavaScriptTest
in favour of Special:JavaScript/qunit/plain. There's a few mobile-related
extensions still using the non-karma mode for qunit testing.

However, none of them make use of the Skin selector, which was mainly a debug
thing I added in the initial version. It no longer makes sense since our tests
now enforce an anti-dependency on skin html and other context. Encouraging
testing in multiple skins in the old UI therefore no longer makes sense.

This also fixes one of the most frequent errors in resourceloader logs and gets
rid of an ugly hack in Resources.php that causes a small amount of overhead
in ResourceLoader::__construct().

> MessageBlobStore failed to find skinname-fallback
> MessageBlobStore failed to find skinname-apioutput

Change-Id: Idaacf718703883c6a7e83a17ccd3f41ebdca53d1