Further improve java-functions to work without JAVA_HOME

[mizdebsk]

• Remove JAVA_HOME check from check_java_env function - follow up on Remove the test for JAVA_HOME and error if it is not set #119
• Obsolete public functions set_jvm and set_jvm_dirs - they do nothing
• Remove private function _set_java_home

CC @fridrich

[mizdebsk]

Also search for JAVACMD under JAVA_HOME only if it's set - otherwise ${JAVA_HOME}/bin/java expands to just /bin/java and likely matches some Java, possibly different one than from PATH via which.

[fridrich]

I also think that this would actually justify a 6.3.2 if all the scripts are cleaned up. It is such an improvement.

[mizdebsk]

I also think that this would actually justify a 6.3.2

I will make a release with current fixes/improvements.

There is another one I'd like to do (#121), but it's a potentially breaking change, so I'll defer it to 7.0.0