Revert "types/key: add MachinePrivate and MachinePublic."

Broke the tailscale control plane due to surprise different serialization.

This reverts commit 4fdb88efe1.

tailcfg/tailcfg.go

@@ -77,6 +77,9 @@ func (u StableNodeID) IsZero() bool {
 	return u == ""
 }
 
+// MachineKey is the curve25519 public key for a machine.
+type MachineKey [32]byte
+
 // NodeKey is the curve25519 public key for a node.
 type NodeKey [32]byte
 
@@ -154,7 +157,7 @@ type Node struct {
 
 	Key        NodeKey
 	KeyExpiry  time.Time
-	Machine    key.MachinePublic
+	Machine    MachineKey
 	DiscoKey   DiscoKey
 	Addresses  []netaddr.IPPrefix // IP addresses of this Node directly
 	AllowedIPs []netaddr.IPPrefix // range of IP addresses to route to this node
@@ -1075,6 +1078,11 @@ type Debug struct {
 	DisableUPnP opt.Bool `json:",omitempty"`
 }
 
+func (k MachineKey) String() string                   { return fmt.Sprintf("mkey:%x", k[:]) }
+func (k MachineKey) MarshalText() ([]byte, error)     { return keyMarshalText("mkey:", k), nil }
+func (k MachineKey) HexString() string                { return fmt.Sprintf("%x", k[:]) }
+func (k *MachineKey) UnmarshalText(text []byte) error { return keyUnmarshalText(k[:], "mkey:", text) }
+
 func appendKey(base []byte, prefix string, k [32]byte) []byte {
 	ret := append(base, make([]byte, len(prefix)+64)...)
 	buf := ret[len(base):]
@@ -1108,6 +1116,9 @@ func (k *NodeKey) UnmarshalText(text []byte) error { return keyUnmarshalText(k[:
 // IsZero reports whether k is the zero value.
 func (k NodeKey) IsZero() bool { return k == NodeKey{} }
 
+// IsZero reports whether k is the zero value.
+func (k MachineKey) IsZero() bool { return k == MachineKey{} }
+
 func (k DiscoKey) String() string                   { return fmt.Sprintf("discokey:%x", k[:]) }
 func (k DiscoKey) MarshalText() ([]byte, error)     { return keyMarshalText("discokey:", k), nil }
 func (k *DiscoKey) UnmarshalText(text []byte) error { return keyUnmarshalText(k[:], "discokey:", text) }