Remove leftover code for header diagnostic info bar

ClangCodeModel used to show an info bar about errors in headers in the
editor, but that was removed. Remove the remaining infrastructure that
was used to show them.

Amends b52fac7148

Change-Id: Id89f2bcb7778245f468923d29c577bf1ca68ee75
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/cppeditor/baseeditordocumentprocessor.h

@@ -89,16 +89,12 @@ public:
 
     QString filePath() const { return m_filePath; }
 
-public:
-    using HeaderErrorDiagnosticWidgetCreator = std::function<QWidget*()>;
-
 signals:
     // Signal interface to implement
     void projectPartInfoUpdated(const ProjectPartInfo &projectPartInfo);
 
     void codeWarningsUpdated(unsigned revision,
                              const QList<QTextEdit::ExtraSelection> &selections,
-                             const HeaderErrorDiagnosticWidgetCreator &creator,
                              const TextEditor::RefactorMarkers &refactorMarkers);
 
     void ifdefedOutBlocksUpdated(unsigned revision,