Remove rogue exit(0) that shuts down JVM

For normal JRuby use, we let the main script finish and Main.main
exits. If non-daemon threads remain running, they will keep the
JVM alive. However a change for CRaC support accidentally
introduced a hard exit(0) that is always encountered, even if the
script did not request a hard exit using `exit` or similar Ruby
calls.

This patch removes the hard exit(0) so that "casual" exit from the
main script does not force the JVM to shut down. The runtime will
still be torn down as usual, but non-daemon threads can keep the
JVM alive after JRuby itself has exited.

Fixes jruby#8416

core/src/main/java/org/jruby/Main.java

@@ -199,8 +199,6 @@ public static void main(String[] args) {
             if (status.isExit()) {
                 System.exit(status.getStatus());
             }
-
-            System.exit(0);
         }
         catch (RaiseException ex) {
             System.exit( handleRaiseException(ex) );