Remove support for GCJ

[jelovirt]

Remove support for GCJ. According to GCJ Wikipedia page

As of 2015, there were no new developments announced from GCJ and the product was in maintenance mode, with open-source Java toolchain development mostly happening within OpenJDK. GCJ was removed from the GCC trunk on September 30, 2016. Announcement of its removal was made with the release of the GCC 7.1, which does not contain it.

While the jing-trang project is not under active development and GCJ support doesn't really harm the codebase, it is legacy feature that is likely not needed anywhere anymore. The code is accessible in Git for those who need it, but the main line should no longer have GCJ support.

AFAICT, GCJ only support Java 1.5 and jing-trang README says Java 8 or above is needed. Thus also the current code is unlikely compatible with GCJ anymore.

[jelovirt]

Relates to #1 from 2008 by J. Clark

Also the question has to be asked of whether there is sufficient benefit to a gcj build to make the effort worthwhile. I doubt it will run much faster.