LanguageClient: improve Server startup

Starting a client does not need to be done via
LanguageClientManager::startClient anymore but can be done via
Client::start()

Change-Id: I571e96b6ad7d64786a345edf5dbb38208b9a1d12
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/client.h

@@ -151,7 +151,7 @@ public:
         const LanguageServerProtocol::DocumentUri &uri,
         const QTextCursor &cursor) const;
 
-    bool start();
+    void start();
     bool reset();
 
     void log(const QString &message);