Editor: delete assist interface in lsp/clangd support

The complete memory management in the code assistant needs an overhaul.
For now just delete or at least track the assist interface with scoped
pointers.

Fixes: QTCREATORBUG-28408
Change-Id: I0bd4cfaa36a660b6fd5bb467af3b13414ed76e63
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/clangcodemodel/clangdcompletion.cpp

@@ -417,6 +417,7 @@ IAssistProposal *CustomAssistProcessor::perform(const AssistInterface *interface
         break;
     }
     }
+    delete interface;
     GenericProposalModelPtr model(new GenericProposalModel);
     model->loadContent(completions);
     const auto proposal = new GenericProposal(m_position, model);