[OPENJDK-3033] drop singleton-jdk from openjdk-21

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

Signed-off-by: Jonathan Dowland <jdowland@redhat.com>

ubi9-openjdk-21.yaml

@@ -54,7 +54,6 @@ modules:
     version: "3.8.17"
   - name: jboss.container.java.s2i.bash
   - name: jboss.container.util.tzdata
-  - name: jboss.container.java.singleton-jdk
 
 help:
   add: true