Remove everything related to nonexistent $wgExcludeFromThumbnailPurge

It has no documentation page on MediaWiki wiki and doesn't grep in MediaWiki core.

Change-Id: I971ea7ad9b0d0518927edd4a082696a29a75ee74
Merge "Remove everything related to nonexistent $wgExcludeFromThumbnailPurge"