Don't wrap DefaultKeychain with refreshes (google#1791)

The heuristic only works well if the upstream keychain doesn't do any
caching. This is usually the case, but I've run into a couple situations
where the hardcoded 5 minutes does not overlap well with certain cred
helper implementations.

Undoing that wrap allows callers to set a time that makes sense for
them. With the wrap, there's not any direct reference to
DefaultKeychain, which feels... bad.

pkg/authn/keychain.go

@@ -53,7 +53,7 @@ type defaultKeychain struct {
 
 var (
 	// DefaultKeychain implements Keychain by interpreting the docker config file.
-	DefaultKeychain = RefreshingKeychain(&defaultKeychain{}, 5*time.Minute)
+	DefaultKeychain = &defaultKeychain{}
 )
 
 const (