Get rid of logic that clears sync point when layer is hidden

For app transitions that go to Launcher, Launcher is initially
hidden, including the leash controlling it as well as the actual
WSA surface. Since the barrier layer of the leash of the closing
app is Launcher WSA, which is a child of the Launcher leash, the
sync point will be cleared because it's not yet shown, as the
showing itself is synced to WSA Launcher surface. However that
destroys the synchronization, leading to jank in various ways.

The justification why this code was added doesn't make sense to
me: SurfaceView usually synchronizes with Activity main window
surfaces. These get destroyed as soon as the activity goes away.
We don't think that logic is needed at all.

Test: Open app, press back, observe no jank
Test: Play video in app
Fixes: 126340236
Change-Id: Ib70d3bbdce1c7206487359213803e3e59d332400