Drop some long-time deprecated entities

While doing the kf6-full entity, I stumbled over those with deprecation notes. Given the major version bump, perhaps a good time to clean-up?

Some in the deprecated section of src/customization/entities/general.entities I kept, as I found them still in use:

• kde-ftp: while outdated even used in this repo's own docbooks
• package: also had hits across docs (edit: sometimes with local redefines or not matching the context?)
• kdenlive, kdesvn, kuickshow: why are they there, still deprecated?
• Netscape: used at least by konqueror & kmail docs

@ltoscano @aacid @lueck