Remove some unused files from skins/common/

Grepping doesn't point to any usages, and they don't appear in logs
from bug 69277 comment 11.

add.png
Arr_.png
closewindow.png
diffunderline.gif
link_icon.gif
remove.png
tooltip_icon.png
Zoom_sans.gif

Bug: 69277
Change-Id: I09c0c13b353366e83124e7ffa1fa02eeada76729
Merge "Remove some unused files from skins/common/"