Delete skins/common/images/closewindow19x19.png

Unused in core, unused in extensions (as far as I can tell) and unused
on-wiki (as far as I can tell).

There are still requests for it (according to bug 69277 comment 11), but
we can't do anything about that if we can't tell where they come from.

Bug: 69277
Change-Id: I2cf158229955c93ecbf29c83d04df121e4b4addc