katecompletionmodel fix Use QHash<K,T> instead of QMap<K,T>
Fix clazy issue "ktexteditor/src/completion/katecompletionmodel.cpp:861:48: warning: Use QHash<K,T> instead of QMap<K,T> when K is a pointer [clazy-qmap-with-pointer-key]".
https://github.com/KDE/clazy/blob/1.11/docs/checks/README-qmap-with-pointer-key.md