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.

docshell/base/nsDocShell.cpp

@@ -956,7 +956,6 @@ NS_INTERFACE_MAP_BEGIN(nsDocShell)
   NS_INTERFACE_MAP_ENTRY(nsIAuthPromptProvider)
   NS_INTERFACE_MAP_ENTRY(nsILoadContext)
   NS_INTERFACE_MAP_ENTRY(nsIWebShellServices)
-  NS_INTERFACE_MAP_ENTRY(nsILinkHandler)
   NS_INTERFACE_MAP_ENTRY(nsIClipboardCommands)
   NS_INTERFACE_MAP_ENTRY(nsIDOMStorageManager)
   NS_INTERFACE_MAP_ENTRY(nsINetworkInterceptController)
@@ -14058,7 +14057,7 @@ OnLinkClickEvent::OnLinkClickEvent(nsDocShell* aHandler,
 {
 }
 
-NS_IMETHODIMP
+nsresult
 nsDocShell::OnLinkClick(nsIContent* aContent,
                         nsIURI* aURI,
                         const char16_t* aTargetSpec,
@@ -14110,7 +14109,7 @@ nsDocShell::OnLinkClick(nsIContent* aContent,
   return NS_DispatchToCurrentThread(ev);
 }
 
-NS_IMETHODIMP
+nsresult
 nsDocShell::OnLinkClickSync(nsIContent* aContent,
                             nsIURI* aURI,
                             const char16_t* aTargetSpec,
@@ -14276,7 +14275,7 @@ nsDocShell::OnLinkClickSync(nsIContent* aContent,
   return rv;
 }
 
-NS_IMETHODIMP
+nsresult
 nsDocShell::OnOverLink(nsIContent* aContent,
                        nsIURI* aURI,
                        const char16_t* aTargetSpec)
@@ -14329,7 +14328,7 @@ nsDocShell::OnOverLink(nsIContent* aContent,
   return rv;
 }
 
-NS_IMETHODIMP
+nsresult
 nsDocShell::OnLeaveLink()
 {
   nsCOMPtr<nsIWebBrowserChrome> browserChrome(do_GetInterface(mTreeOwner));