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.go

@@ -8,7 +8,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"iter"
 	"net"
 	"strconv"
 )
@@ -84,11 +83,6 @@ type StateStore interface {
 	// instead, which only writes if the value is different from what's
 	// already in the store.
 	WriteState(id StateKey, bs []byte) error
-	// All returns an iterator over all StateStore keys. Using ReadState or
-	// WriteState is not safe while iterating and can lead to a deadlock.
-	// The order of keys in the iterator is not specified and may change
-	// between runs.
-	All() iter.Seq2[StateKey, []byte]
 }
 
 // WriteState is a wrapper around store.WriteState that only writes if