Remove TaskFragment Shell transition collection when startActivity

The old logic was introduced when we were not sure when the TaskFragment
may change during startActivity. After [1], we now collect TaskFragment
when the organizer has requested to change it. Thus the old logic should
not be needed anymore.

[1]: d00861c46e

Fix: 258095975
Test: verify the opening ActivityEmbedding split app has no flicker
Change-Id: I3e139e216622ef6ffc5506b70e3962eb4a609dfb

services/core/java/com/android/server/wm/ActivityStarter.java

@@ -3037,12 +3037,6 @@ class ActivityStarter {
                 newParent = candidateTf;
             }
         }
-        if (newParent.asTask() == null) {
-            // only collect task-fragments.
-            // TODO(b/258095975): we probably shouldn't ever collect the parent here since it isn't
-            //                    changing. The logic that changes it should collect it.
-            newParent.mTransitionController.collect(newParent);
-        }
         if (mStartActivity.getTaskFragment() == null
                 || mStartActivity.getTaskFragment() == newParent) {
             newParent.addChild(mStartActivity, POSITION_TOP);