Clang: Do not show diagnostic category in inline annotations and issues pane

For build system diagnostics they are not displayed either.

Also, the error/warning is already encoded in the icon and the color.

However, the category is still shown in the tooltip.

Change-Id: I190ab17691c32786cefc20d058010c65cda5ace9
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangutils.h

@@ -56,6 +56,8 @@ bool isProjectPartLoaded(const CppTools::ProjectPart::Ptr projectPart);
 QString projectPartIdForFile(const QString &filePath);
 int clangColumn(const QTextBlock &lineText, int cppEditorColumn);
 
+QString diagnosticCategoryPrefixRemoved(const QString &text);
+
 CPlusPlus::Icons::IconType iconTypeForToken(const ClangBackEnd::TokenInfoContainer &token);
 
 } // namespace Utils