remove native Bullet from the Engine

[stephengold]

As discussed at the Forum.

[tlf30]

The code in jme3-bullet/src/common/java/com/jme3/bullet still exists. Is this intentional/due to jme-jbullet?
https://github.com/jMonkeyEngine/jmonkeyengine/tree/gsw/jme3-bullet/src/common/java/com/jme3/bullet

Just wondering as the gradle build script is now gone in that module.

[stephengold]

Yeah, that's intentional. As long as I can remember, jme3-jbullet has used the "src/common" files from jme3-bullet, and that dependency is still apparent here:

jmonkeyengine/jme3-jbullet/build.gradle

Line 9 in ace22d1

srcDir '../jme3-bullet/src/common/java'

(The "src/main" files from jme3-bullet are deleted by this PR.)