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/mem/store_mem.go

@@ -7,7 +7,6 @@ package mem
 import (
 	"bytes"
 	"encoding/json"
-	"iter"
 	"sync"
 
 	xmaps "golang.org/x/exp/maps"
@@ -86,16 +85,3 @@ func (s *Store) ExportToJSON() ([]byte, error) {
 	}
 	return json.MarshalIndent(s.cache, "", "  ")
 }
-
-func (s *Store) All() iter.Seq2[ipn.StateKey, []byte] {
-	return func(yield func(ipn.StateKey, []byte) bool) {
-		s.mu.Lock()
-		defer s.mu.Unlock()
-
-		for k, v := range s.cache {
-			if !yield(k, v) {
-				break
-			}
-		}
-	}
-}