Issue #2721 - Remove nsILinkHandler.

This interface inly has a single implementation behind it, which is also
only used in 2 places after the previous commit. That's a lot of
additional complexity and compiler indirection for no good reason.
This change removes the interface and uses direct nsDocShell::Cast calls
instead of going through the interface in the few places left now that
we no longer build on a presentation context for links.

layout/base/nsPresContext.h

@@ -52,7 +52,6 @@ class nsITheme;
 class nsIContent;
 class nsIFrame;
 class nsFrameManager;
-class nsILinkHandler;
 class nsIAtom;
 class nsIRunnable;
 class gfxUserFontEntry;