[JENKINS-74990] Prevent stack overflow / Revert "Add icons for search"

https://issues.jenkins.io/browse/JENKINS-74990 notes that when a folder
definition includes the XML:

```
<icon class="jenkins.branch.MetadataActionFolderIcon" plugin="[email protected]_a_3da_2eb_db_4">
  <owner class="com.cloudbees.hudson.plugins.folder.Folder" reference="../.."/>
</icon>
```

then the 6.971.v9a_984fd08864 release of the plugin reports a stack overflow.

Revert the change so that there is enough time for diagnosis without
the pressure of users reporting and diagnosing bugs.

This reverts commit c9fe2ab.

src/main/java/com/cloudbees/hudson/plugins/folder/AbstractFolder.java

@@ -101,7 +101,6 @@
 import jenkins.model.ProjectNamingStrategy;
 import jenkins.model.TransientActionFactory;
 import net.sf.json.JSONObject;
-import org.jenkins.ui.icon.IconSpec;
 import org.kohsuke.accmod.Restricted;
 import org.kohsuke.accmod.restrictions.Beta;
 import org.kohsuke.accmod.restrictions.NoExternalUse;
@@ -135,7 +134,7 @@
  */
 @SuppressWarnings({"unchecked", "rawtypes"}) // mistakes in various places
 @SuppressFBWarnings("DMI_RANDOM_USED_ONLY_ONCE") // https://github.com/spotbugs/spotbugs/issues/1539
-public abstract class AbstractFolder<I extends TopLevelItem> extends AbstractItem implements TopLevelItem, ItemGroup<I>, ModifiableViewGroup, StaplerFallback, ModelObjectWithChildren, StaplerOverridable, IconSpec {
+public abstract class AbstractFolder<I extends TopLevelItem> extends AbstractItem implements TopLevelItem, ItemGroup<I>, ModifiableViewGroup, StaplerFallback, ModelObjectWithChildren, StaplerOverridable {
 
     /**
      * Our logger.
@@ -306,11 +305,6 @@ protected AbstractFolderViewHolder newFolderViewHolder() {
         return new DefaultFolderViewHolder(views, null, newDefaultViewsTabBar());
     }
 
-    @Override
-    public String getIconClassName() {
-        return icon.getIconClassName();
-    }
-
     protected FolderIcon newDefaultFolderIcon() {
         return new StockFolderIcon();
     }