Doc: Remove incorrect, duplicate namespace in documentation config

The Qt version tag is expected to appear in the .qhp namespace
configuration. Remove the duplicate config line with incorrect
`dev` version string.

Fixes: QTBUG-151719
Pick-to: 6.12
Change-Id: Icb007f49ec903e9d252fef17e52d1d8f9d2b6a40
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>