wgengine/magicsock: adapt to wireguard-go without UpdateDst

22507adf54 stopped relying on
our fork of wireguard-go's UpdateDst callback.
As a result, we can unwind that code,
and the extra return value of ReceiveIPv{4,6}.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/magicsock/legacy.go

@@ -448,10 +448,6 @@ func (a *addrSet) SrcIP() net.IP       { return nil }
 func (a *addrSet) SrcToString() string { return "" }
 func (a *addrSet) ClearSrc()           {}
 
-func (a *addrSet) UpdateDst(new *net.UDPAddr) error {
-	return nil
-}
-
 // updateDst records receipt of a packet from new. This is used to
 // potentially update the transmit address used for this addrSet.
 func (a *addrSet) updateDst(new *net.UDPAddr) error {