kde-misc/kdiff3: Drop USE=debug,handbook,kde,qt4

kde-misc/kdiff3: Drop USE=debug,handbook,kde,qt4

kde-frameworks/kdelibs is deprecated and going to be removed.
Plasma users are served well by the kf5-based snapshot, everyone else
probably does not care about USE=kde in this version of kdiff3 anymore.

- Remove obsolete REQUIRED_USE
- Bump to EAPI 6
- Drop kde4-base.eclass
- sed in place and add missing || die

KDE_LINGUAS and USE=handbook only ever worked with USE=kde enabled.
The remaining bits and pieces install exactly as USE=-kde did before.

Gentoo-bug: 629018
Package-Manager: Portage-2.3.8, Repoman-2.3.3