Reduce usages of com.google.common.collect in favor of native Java functionality (#5464)

* Reduce usages of `com.google.common.collect` in favor of native Java functionality

* Copy original list to preserve original behavior

core/src/main/java/hudson/PluginWrapper.java

@@ -24,7 +24,6 @@
  */
 package hudson;
 
-import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Sets;
 import hudson.PluginManager.PluginInstanceStore;
 import hudson.model.AdministrativeMonitor;
@@ -220,7 +219,7 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
      * The core can depend on a plugin if it is bundled. Sometimes it's the only thing that
      * depends on the plugin e.g. UI support library bundle plugin.
      */
-    private static Set<String> CORE_ONLY_DEPENDANT = ImmutableSet.copyOf(Collections.singletonList("jenkins-core"));
+    private static Set<String> CORE_ONLY_DEPENDANT = Collections.singleton("jenkins-core");
 
     /**
      * Set the list of components that depend on this plugin.