The port misc/kdehier4, which defines the kdeprefix component, has been unused since 2014 and expired on 2016-03-09
This patch removes that port, cleans up bsd.kde4.mk (also for the unreferenced kdehier component) and removes kdeprefix from USE_KDE4 from all ports that still referenced it.
See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209014 (which only contains a partial patch)