[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.

unittests/SourceKit/SwiftLang/CursorInfoTest.cpp

@@ -68,12 +68,11 @@ public:
   }
 
   CursorInfoTest()
-      : Ctx(*new SourceKit::Context(getSwiftExecutablePath(),
-                                    getRuntimeLibPath(),
-                                    /*diagnosticDocumentationPath*/ "",
-                                    SourceKit::createSwiftLangSupport,
-                                    [](SourceKit::Context &Ctx){ return nullptr; },
-                                    /*dispatchOnMain=*/false)) {
+      : Ctx(*new SourceKit::Context(
+            getSwiftExecutablePath(), getRuntimeLibPath(),
+            SourceKit::createSwiftLangSupport,
+            [](SourceKit::Context &Ctx) { return nullptr; },
+            /*dispatchOnMain=*/false)) {
     INITIALIZE_LLVM();
     // This is avoiding destroying \p SourceKit::Context because another
     // thread may be active trying to use it to post notifications.