wgengine/wgcfg,wgengine,ipn/ipnlocal: remove Peers from wgcfg.Config

The wireguard-go device now learns its peer set solely from the live
per-peer config source that LocalBackend installs with
Engine.SetPeerConfigFunc, backed by the route manager. Peers are
created lazily on first packet and converged per peer with
Engine.SyncDevicePeer, so the full-peer-list snapshot in wgcfg.Config
and the diff-and-reconfigure machinery around it (wgcfg.Peer,
ReconfigDevice, and the engine's full device sync in
maybeReconfigWireguardLocked) are dead weight: they duplicated state
that the route manager already owns and forced every netmap change to
rebuild and rehash the entire peer list.

Delete the Peers field and the Peer type from wgcfg, along with
ReconfigDevice and maybeReconfigWireguardLocked. Engine.Reconfig no
longer does any device peer work; it only manages the private key,
addresses, and the non-peer subsystems. Full-netmap application converges the device by
syncing exactly the peers whose routes the route manager reports as
changed or removed.

Updates #12542

Change-Id: Ic776e42cfaa5be6b9329b3d381d5cbde17d7078b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/deephash/tailscale_types_test.go

@@ -86,11 +86,6 @@ func getVal() *tailscaleTypes {
 	return &tailscaleTypes{
 		&wgcfg.Config{
 			Addresses: []netip.Prefix{netip.PrefixFrom(netip.AddrFrom16([16]byte{3: 3}).Unmap(), 5)},
-			Peers: []wgcfg.Peer{
-				{
-					PublicKey: key.NodePublic{},
-				},
-			},
 		},
 		&router.Config{
 			Routes: []netip.Prefix{