Stop using pack200 because it has been deprecated and removed

Deprecated in Java 11 https://openjdk.java.net/jeps/336 and removed
in Java 14 https://openjdk.java.net/jeps/367.

The main problem for us now is that the packed JARs don't get
signed when using maven-jarsigner-plugin, while the unpacked
ones do. Simplest solution is to let go of pack200 already.

gitlab simantics/platform#468