Erase border that appears with TMP (#528)

I've checked conflict as much as possible, but I'm afraid my oversights.

note: This is due to tabmixplus/chrome/content/tab/tab.js, backgroundRule set linear-gradient to background-image.
note2: I use a rule 'not([selected="true"])', because treestyletab/skin/classic/treestyletab/sidebar/sidebar-base.css set
  background to '.tabbrowser-tab[selected="true"]'.