QtHelp: Drop stale namespaces when registering a file again

A qch file holds exactly one namespace. When the docs are rebuilt in
place after a version bump (e.g. 6.12 -> 6.13), the file at the same
path gets a new namespace, but the collection kept the row of the old
one. Both rows then pointed at the same file, which showed up as two
identical versions in the version selection and as a mismatch between
the registered and the watched docs. Registering a file now
unregisters other namespaces that point to the same path.

Pick-to: 6.12
Change-Id: Iea839ad17502086ec9d413e848e9ed4817773901
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>