Clean up after java/openjdk6 and java/openjdk6-jre removal

java/openjdk6 support was removed from Mk/bsd.java.mk (r512662) and
java/openjdk6 and java/openjdk6-jre were removed from the ports tree
(r512663). Now this patch completely removes remaining stuff from the
ports tree.

PR: 241953 (exp-run)
Reviewed by: glewis
Approved by: portmgr (antoine)
Differential Revision: https://reviews.freebsd.org/D22342