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/awsstore/store_aws.go

@@ -10,7 +10,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"iter"
 	"net/url"
 	"regexp"
 	"strings"
@@ -254,7 +253,3 @@ func (s *awsStore) persistState() error {
 	_, err = s.ssmClient.PutParameter(context.TODO(), in)
 	return err
 }
-
-func (s *awsStore) All() iter.Seq2[ipn.StateKey, []byte] {
-	return s.memory.All()
-}