Doc: Remove incorrect see-also entry

QDoc cross-links notifier signal documentation automatically with the
associated property, so there's no need for an explicit \sa entry.

Fixes
    (qdoc) warning: Can't link to 'httpUserAgent()'

Pick-to: 6.12
Change-Id: I99f76df41024425d5ee9e1dbec7e9ea767cdc803
Reviewed-by: Jerome Pasion <jerome.pasion@qt.io>