[SourceKit] Remove diagnostics path calculation

The diagnostic group documentation now point to the swift.org URL rather
than the toolchain path, so it no longer needs to be passed all the way
through sourcekitd.

Resolves rdar://151500502.

(cherry picked from commit 381684a389)

tools/SourceKit/lib/SwiftLang/SwiftASTManager.h

@@ -259,8 +259,7 @@ public:
                            std::shared_ptr<RequestTracker> ReqTracker,
                            std::shared_ptr<swift::PluginRegistry> Plugins,
                            StringRef SwiftExecutablePath,
-                           StringRef RuntimeResourcePath,
-                           StringRef DiagnosticDocumentationPath);
+                           StringRef RuntimeResourcePath);
   ~SwiftASTManager();
 
   SwiftInvocationRef getTypecheckInvocation(ArrayRef<const char *> Args,