remove boolean return from unmountCallback

src/base-element.js

@@ -540,13 +540,8 @@ export class BaseElement {
 
   /**
    * Unload heavy elements, remove global listeners, etc.
-   *
-   * @return {boolean} Return `true` if the element should be remounted
-   * the next time it's displayed.
    */
-  unmountCallback() {
-    return false;
-  }
+  unmountCallback() {}
 
   /**
    * Subclasses can override this method to opt-in into receiving additional

src/custom-element.js

@@ -714,12 +714,9 @@ function createBaseCustomElementClass(win, elementConnectedCallback) {
       const scheduler = getSchedulerForDoc(this.getAmpDoc());
       scheduler.unschedule(this);
 
-      // Try to unmount. Not every element has been mounted, can be unmounted
-      // or has anything to unmount.
-      const reMount =
-        !this.mountPromise_ || !this.impl_ || this.impl_.unmountCallback();
-      if (!reMount) {
-        return;
+      // Try to unmount if the element has been built already.
+      if (this.isBuilt()) {
+        this.impl_.unmountCallback();
       }
 
       // Complete unmount and reset the state.

testing/element-v1.js

@@ -70,6 +70,15 @@ const RULES = [
       return !hasCallback;
     },
   },
+  {
+    name: 'Must not have resumeCallback',
+    test: (implClass) => {
+      const hasCallback =
+        implClass.prototype.resumeCallback !==
+        BaseElement.prototype.resumeCallback;
+      return !hasCallback;
+    },
+  },
 
   {
     name: 'If load==true, must also have ensureLoaded',