[Openjdk 3029] no singleton jdk required for openjdk-21

[jmtd]

This used to be required to remove the system JDK, as pulled in by Maven, prior to the introduction of the maven-openjdk21 package.

Test coverage exists in modules/jdk/tests/features/openjdk.feature

https://issues.redhat.com/browse/OPENJDK-3029

[jmtd]

Since the source change is a one-liner, for clarity here's the impact it has on the generated Dockerfile

@@ -283,22 +283,12 @@
 ###### /
 ###### END module 'jboss.container.java.s2i.bash:1.0'

-###### START module 'jboss.container.java.singleton-jdk:1.0'
-###### \
-        # Copy 'jboss.container.java.singleton-jdk' module content
-        COPY modules/jboss.container.java.singleton-jdk /tmp/scripts/jboss.container.java.singleton-jdk
-        # Custom scripts from 'jboss.container.java.singleton-jdk' module
-        USER root
-        RUN [ "sh", "-x", "/tmp/scripts/jboss.container.java.singleton-jdk/configure.sh" ]
-###### /
-###### END module 'jboss.container.java.singleton-jdk:1.0'
-
 ###### START image 'ubi8/openjdk-21:1.19'
 ###### \
         # Set 'ubi8/openjdk-21' image defined environment variables

And the code that isn't called anymore is https://github.com/jboss-container-images/openjdk/blob/ubi8/modules/singleton-jdk/configure.sh