Cleanup a4a/performance's private fields

All three seem unused.

ads/google/a4a/performance.js

@@ -131,12 +131,6 @@ export class GoogleAdLifecycleReporter extends BaseLifecycleReporter {
  constructor(win, element, slotId) {
  super();
 
- /** @private {!Window} @const */
- this.win_ = win;
-
- /** @private {!Element} @const */
- this.element_ = element;
-
  /** @private {string} @const */
  this.namespace_ =
  element.getAttribute('data-a4a-upgrade-type') ? 'a4a' : 'amp';
@@ -147,9 +141,6 @@ export class GoogleAdLifecycleReporter extends BaseLifecycleReporter {
  /** @private {number} @const */
  this.correlator_ = getCorrelator(win, /* opt_cid */ undefined, element);
 
- /** @private {string} @const */
- this.slotName_ = this.namespace_ + '.' + this.slotId_;
-
  // Contortions to convince the type checker that we're type-safe.
  let initTime;
  const scratch = getTimingDataSync(win, 'navigationStart') || Date.now();