Html: Don't accept paths relative to skins/common/images/ in infoBox()

There are no files worth referencing there anymore.

No usages found in extensions, only one usage in core already didn't
depend on that behavior.

Change-Id: If1fd68a3a7355c8195fd2aad86e7584a5772bd4c
Merge "Html: Don't accept paths relative to skins/common/images/ in infoBox()"