DHCPv6 P flag: remove onNewPrefix callback.

Moving all logic of processing new prefix with P flag to
IpClientLinkObserver, therefore, we don't need this callback anymore.

Flag: EXEMPT guarded by an legacy experiment flag
Bug: 315069348
Test: TH
Change-Id: I24489625735cace3bb1a95b557b467c8a4b6eaae
2 files changed