Simplify User-Agent code to remove KService/KParts dependency

I use the DAV resource to see the effect on the actual User-Agent being sent. Before:

"User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.34 (KHTML, like Gecko) akonadi_davgroupware_resource_0/5.16.90 (21.03.80) Safari/534.34"

After:

"User-Agent: Mozilla/5.0 (X11; Linux x86_64) KIO/5.81 akonadi_davgroupware_resource_0/5.16.90 (21.03.80)"

Since we're not rendering HTML with KIO anymore, I guess it makes sense to simplify it by removing all HTML-related stuff.

KF6 task: https://phabricator.kde.org/T14307