Remove maintenance script dumpSisterSites.php

Remove maintenance script dumpSisterSites.php

This script would perhaps better belong in the SisterSites extension, as noted
on the extension's MW.org description page. However, that extension was never
actually developed to a usable state (first commit was r16390, no substantial
changes since) or imported into Git, so apparently the feature (or at least
its implementation) is of only limited, possibly historical interest.

Change-Id: Ia4c3b0316815f06b7e82f013edfd1798ed07d9a3