Remove $wgJavaScriptTestConfig

Remove $wgJavaScriptTestConfig

The config variable itself and the documentation property were added
in MediaWiki 1.19 (r107919 / c447423593).

The testswarm-injectjs propert was added in MediaWiki 1.20 (5e590be3d6).

We never actually ended up using TestSwarm, and this variable
is not used anywhere I can see.

Having a configuration variable for a documentation page seems
odd. I can't find another instance of this. As it's tied to development
(not for users of the wiki), link to mediawiki.org direcly.

Change-Id: Ib16607683a293b6d6661ed0411dad9a3ff551a08