ClangCodeModel: Bump minimum clangd version

... and throw away old workarounds.
The current clangd version, to be shipped with Qt Creator 14, is 18.1, so
we can safely bump the minimum version to 17.

Change-Id: I74fd5997196d774b6c47dcb522284953ef82ad9c
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdsemantichighlighting.h

@@ -33,10 +33,7 @@ void doSemanticHighlighting(
         const Utils::FilePath &filePath,
         const QList<LanguageClient::ExpandedSemanticToken> &tokens,
         const QString &docContents,
-        const ClangdAstNode &ast,
-        const QPointer<TextEditor::TextDocument> &textDocument,
         int docRevision,
-        const QVersionNumber &clangdVersion,
         const TaskTimer &taskTimer
         );