Delete skins/common/images/button_template.png

Unused in core, as well as extensions and skins.

I have removed the two usages on Wikimedia wikis (found by mwgrep)
which must have been the cause of the 97 requests for this file per
bug 69277 comment 11:

* https://de.wikipedia.org/wiki/MediaWiki:Gadget-Vorlagenmeister.css
* https://hu.wikipedia.org/wiki/MediaWiki:Gadget-sablonmester.css

Bug: 69277
Change-Id: I9d151e1fbc2da948b5265da731496d4178f51039