nixpkgs: remove all Java 7 expressions

JDK 7 was technically EOL'd a while ago, although RedHat etc are still
doing updates I believe. However, JDK 8 is the default in the tree and
really used everywhere, and JDK 7 isn't seeing many updates by current maintainers, so dropping it seems appropriate.

Signed-off-by: Austin Seipp <aseipp@pobox.com>