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]: d00861c4

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