ipn/store: make StateStore.All optional (#16409)

This method is only needed to migrate between store.FileStore and
tpm.tpmStore. We can make a runtime type assertion instead of
implementing an unused method for every platform.

Updates #15830

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

ipn/store/kubestore/store_kube.go

@@ -7,7 +7,6 @@ package kubestore
 import (
 	"context"
 	"fmt"
-	"iter"
 	"log"
 	"net"
 	"os"
@@ -429,7 +428,3 @@ func sanitizeKey[T ~string](k T) string {
 		return '_'
 	}, string(k))
 }
-
-func (s *Store) All() iter.Seq2[ipn.StateKey, []byte] {
-	return s.memory.All()
-}