Remove legacy layer mirroring

Mirroring is implemented by visiting the layers in the
hierarchy via different parents. Each "visit" called
a traversal path will have an associated layer snapshot but
only a single legacy layer and a single requested layer state.

Flag: EXEMPT flag removal
Bug: 330785038
Test: presubmit
Change-Id: I24cdcd3a38f643d278d194dd288e9196755858d8