tailcfg,etc: remove unused tailcfg.Node.KeepAlive field

The server hasn't sent it in ages.

Updates #cleanup

Change-Id: I9695ab0f074ec6fb006e11faf3cdfc5ca049fbf8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -742,7 +742,6 @@ func (b *LocalBackend) populatePeerStatusLocked(sb *ipnstate.StatusBuilder) {
 			HostName:     p.Hostinfo.Hostname(),
 			DNSName:      p.Name,
 			OS:           p.Hostinfo.OS(),
-			KeepAlive:    p.KeepAlive,
 			LastSeen:     lastSeen,
 			Online:       p.Online != nil && *p.Online,
 			ShareeNode:   p.Hostinfo.ShareeNode(),

ipn/ipnstate/ipnstate.go

@@ -223,9 +223,8 @@ type PeerStatus struct {
 	LastSeen       time.Time // last seen to tailcontrol; only present if offline
 	LastHandshake  time.Time // with local wireguard
 	Online         bool      // whether node is connected to the control plane
-	KeepAlive      bool
-	ExitNode       bool // true if this is the currently selected exit node.
-	ExitNodeOption bool // true if this node can be an exit node (offered && approved)
+	ExitNode       bool      // true if this is the currently selected exit node.
+	ExitNodeOption bool      // true if this node can be an exit node (offered && approved)
 
 	// Active is whether the node was recently active. The
 	// definition is somewhat undefined but has historically and
@@ -437,9 +436,6 @@ func (sb *StatusBuilder) AddPeer(peer key.NodePublic, st *PeerStatus) {
 	if st.InEngine {
 		e.InEngine = true
 	}
-	if st.KeepAlive {
-		e.KeepAlive = true
-	}
 	if st.ExitNode {
 		e.ExitNode = true
 	}