Remove Special:FirstSteps

Now that the TwnMainPage extension is in use, this is no longer needed.

Dependent patch sets:
* TwnMainPage: https://gerrit.wikimedia.org/r/#/c/103483/
* translatewiki: https://gerrit.wikimedia.org/r/#/c/103485/

The code could be revived in a separate extension if someone wants
to maintain it, but from now on, it's no longer in the scope of the
Translate extension.

Change-Id: I52d5959c3a08ad28193936b10401512c713bec27