LanguageClient: remove codeassist proposal handler

Removes noise from the interface. Using specialized clangd processors
that redirect the proposals for testing purposes is an overall cleaner
solution.

Change-Id: Ia4f3d4720765f0533c5530a2a5c7db3cc971e0be
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangcodemodel/clangdclient.h

@@ -77,6 +77,7 @@ public:
             const LanguageServerProtocol::DocumentUri &uri);
 
     void enableTesting();
+    bool testingEnabled() const;
 
 signals:
     void indexingFinished();
@@ -96,6 +97,8 @@ private:
     class FollowSymbolData;
     class VirtualFunctionAssistProcessor;
     class VirtualFunctionAssistProvider;
+    class ClangdFunctionHintProcessor;
+    class ClangdCompletionAssistProcessor;
     class ClangdCompletionAssistProvider;
     Private * const d;
 };