Remove wgSitesCacheFile and rebuildSitesCache.php (unused)

Remove wgSitesCacheFile and rebuildSitesCache.php (unused)

This seems to have been intended as optimization for SiteStore,
but was never used as far as I can tell. Instead, SiteStore is
already cached via LocalServerCache (APC).

Keep the FileBasedSiteLookup class for one release cycle in case
third parties not indexed by MediaWiki Codesearch are using it.

== History

* 2013: Report of high memcached usage by Wikibase via SiteStore.
* 2014: Lazy-load the data in Wikibase (d3f2e99cb6).
* 2014: Implement the file-based cache (via T47532 and 90f6efc360fd5).
* 2015: Use local-server cache (APC), per T58602.

The file-based code was never used. The related task marked
invalid (T47532).

Change-Id: I8e2d9edcf3880149f824cc3de37793ca57435b49