Clear remoteSyncPoints for detached layers.

When a layer gets detached, the remote sync points should get removed so
the buffer layer isn't waiting for that transaction to get applied.
Detached layers can never apply transactions so the buffer layer will
wait forever for the transaction to get applied.

Test: Steps from bug
Test: ChildLayerTest.DetachChildrenWithDeferredTransaction
Fixes: 132125338
Change-Id: I333855a70a40152457f39e953bc7300d696c7c62