Drop unnecessary CSS selectors for a.external[href^="gopher://"]

Drop unnecessary CSS selectors for a.external[href^="gopher://"]

* They don't do anything anyway - gopher:// links are not styled by
  anything else, so they'd just get the same styles as regular
  external links.
* They cause problems in browsers not supporting the attribute
  selector (like IE6), as the browser drops the entire rule, as
  required by the CSS specification.

Also removed some code in monobook/IE60Fixes.css which seems to be
simply a workaround for the issue described above.

Bug: 31495
Change-Id: Ib33d47d5372dd73097bdcef8d00ebd073f6bbd68