Disable Java wrapper build by default.

[tedski]

• Java wrapper build isn't necessary for all Python builds.
• Java wrapper build can now be enabled with ENABLE_JAVA environment
  variable.

[skvark]

There can be only 4 sdist builds because there are exactly 4 different distributions in PyPI, no need to modify the build matrix. The environment variable is not really needed, Java could be just hardcoded off in all builds since this repository is building the Python wrapper, not the Java one.

[tedski]

I wanted to leave the environment variable in case there were current users that needed the Java wrapper from the build. Let me know if you still want me to remove it.

[skvark]

Thanks!