Remove flag map argument from "flagsChanged" event

This encourages reading of specific flags to be done via `flagEnabled`, which
checks for usage of unknown flags.

src/annotator/features.js

@@ -39,7 +39,7 @@ export class FeatureFlags extends TinyEmitter {
     for (let [flag, on] of Object.entries(flags)) {
       this._flags.set(flag, on);
     }
-    this.emit('flagsChanged', this._flags);
+    this.emit('flagsChanged');
   }
 
   /**

src/annotator/test/features-test.js

@@ -32,15 +32,6 @@ describe('FeatureFlags', () => {
       features.update(testFlags);
 
       assert.calledOnce(onFlagsChanged);
-      const flagMap = onFlagsChanged.getCall(0).args[0];
-      assert.instanceOf(flagMap, Map);
-      assert.deepEqual(
-        [...flagMap.entries()],
-        [
-          ['feature_on', true],
-          ['feature_off', false],
-        ]
-      );
     });
 
     it('updates flags returned by `flagEnabled`', () => {