Remove SkinLegacy.php

Remove SkinLegacy.php

Followup to Ia6d73c2d (the great skin removal event of 2013).

The two classes this file defined, SkinLegacy and LegacyTemplate, were
already removed from AutoLoader.php in the aforementioned commit, but
the file itself somehow stayed.

The Nostalgia extension includes its own copy, and I've rewritten
Cologne Blue to use SkinTemplate last year, which means that nothing
needs the legacy classes anymore. I grepped mediawiki/extensions just
in case and there were no hits apart from Nostalgia.

Change-Id: Ib36edff3f9b617357386914980b56fa5b1f6a83b