Remove Named interface (#26913)

`Named` is implemented by every `Method` and future implementations
should implement the method too.

services/auth/sspi_windows.go

@@ -37,7 +37,6 @@ var (
 
 	// Ensure the struct implements the interface.
 	_ Method = &SSPI{}
-	_ Named  = &SSPI{}
 )
 
 // SSPI implements the SingleSignOn interface and authenticates requests