timers: no need to clear keepalive in persistent keepalive

We do this after sending the keepalive anyway.

This is something of a regression, though, since before we'd cancel and
then send, but now we send and then cancel, so it introduces a potential
race, but hopefully that isn't too big of a deal.

Kernel module commit a24b3e6e15ae1ea1291666e5da910caf43eedbaf

timers.go

@@ -134,9 +134,6 @@ func expiredZeroKeyMaterial(peer *Peer) {
 
 func expiredPersistentKeepalive(peer *Peer) {
 	if peer.persistentKeepaliveInterval > 0 {
-		if peer.timersActive() {
-			peer.timers.sendKeepalive.Del()
-		}
 		peer.SendKeepalive()
 	}
 }