ssh,acme: clean up tautological/impossible nil conditions

Change-Id: I4c6b4b6d8dc1e8a9d2ebfb8d350b7617d3cf7949
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/739780
Auto-Submit: Neal Patel <nealpatel@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>

ssh/agent/server.go

@@ -36,7 +36,7 @@ func (s *server) processRequestBytes(reqData []byte) []byte {
 		return []byte{agentFailure}
 	}
 
-	if err == nil && rep == nil {
+	if rep == nil {
 		return []byte{agentSuccess}
 	}