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/languageclientfunctionhint.cpp

@@ -96,7 +96,7 @@ IAssistProposal *FunctionHintProcessor::perform(const AssistInterface *interface
     SignatureHelpRequest request((TextDocumentPositionParams(TextDocumentIdentifier(uri), Position(cursor))));
     request.setResponseCallback([this](auto response) { this->handleSignatureResponse(response); });
     m_client->addAssistProcessor(this);
-    m_client->sendContent(request);
+    m_client->sendMessage(request);
     m_currentRequest = request.id();
     return nullptr;
 }