LanguageClient: reset all assist providers

Instead of just unsetting the function assist provider and quick fix
assist provider reset it to the previous provider.

Change-Id: I46c5c2f14234e3da08480dd103e4634859447d57
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientfunctionhint.h

@@ -33,6 +33,8 @@ class Client;
 
 class FunctionHintAssistProvider : public TextEditor::CompletionAssistProvider
 {
+    Q_OBJECT
+
 public:
     explicit FunctionHintAssistProvider(Client *client);