LanguageServerProtocol: remove IContent

Do not pretend to support multiple message types, while no other
messages are actually implemented by us or even known to exist in the
wild.

Change-Id: I49ee2118b2e10f265ac641c195df8a9e5c97951c
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/languageclient/documentsymbolcache.cpp

@@ -106,7 +106,7 @@ void DocumentSymbolCache::requestSymbolsImpl()
                 self->handleResponse(uri, response);
         });
         m_runningRequests[uri] = request.id();
-        m_client->sendContent(request);
+        m_client->sendMessage(request);
     }
     m_compressedUris.clear();
 }