Nostalgia.php cleanup

* Updated documentation
* Removed pointless $dir = __DIR__
* Removed pointless defined( 'MEDIAWIKI' ) check

Change-Id: Id42c41edd3cab72f6301a1c5d632aa9b4e1acef3
Adjust 'remoteBasePath' to the skins/ directory

Bug: 61256
Change-Id: Ia3bf08de3e99f4f71c0a9412372dbcbf7a8d0e8e