ipn/store: remove a layer of indirection for registering stores (#15986)

Registering a new store is cheap, it just adds a map entry. No need to
lazy-init it with sync.Once and an intermediate slice holding init
functions.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

ipn/store/store_kube.go

@@ -14,10 +14,6 @@ import (
 )
 
 func init() {
-	registerAvailableExternalStores = append(registerAvailableExternalStores, registerKubeStore)
-}
-
-func registerKubeStore() {
 	Register("kube:", func(logf logger.Logf, path string) (ipn.StateStore, error) {
 		secretName := strings.TrimPrefix(path, "kube:")
 		return kubestore.New(logf, secretName)