contrib/znc-import: leave password empty by default

No need for the "!!" hack, an empty password field means that
auth is disabled already.

contrib/znc-import/main.go

@@ -107,8 +107,6 @@ func main() {
 			log.Printf("user %q: updating existing user", username)
 		} else {
 			u = database.NewUser(username)
-			// "!!" is an invalid crypt format, thus disables password auth
-			u.Password = "!!"
 			usersCreated++
 			log.Printf("user %q: creating new user", username)
 		}