QtHelp: Don't fail unregistering index tables without a component

unregisterIndexTable() returned false when the namespace had no entry
in the ComponentMapping table, after it had already removed all other
index data. This happens when a namespace with outdated index tables
fails to be registered again when opening the collection file: the
first unregisterIndexTable() call removes the mapping, and the
following unregisterDocumentation() then fails.

Treat a missing component mapping as nothing left to remove. This is
needed to make unregistering documentation atomic in a follow-up
change, which otherwise would roll back the removal in this case.

Task-number: QTBUG-150686
Pick-to: 6.12 6.8
Change-Id: I0c39cd4df1fc86ae6021fb932e0aeaa106b87d5a
Reviewed-by: Kai Köhne <kai.koehne@qt.io>