txnbuild: remove superfluous signing key in challenge txn test (#2279)

### What

Remove superfluous signing key in challenge transaction test that is used to sign the transaction twice when it only needs to sign the transaction once.

### Why

The key is added twice which was probably the result of a copy and paste from another tests where we test the behavior of signing a transaction twice. It's unnecessary to sign it twice here and having it sign the transaction twice is noise that is irrelevant to the scenario being tested in this test.

txnbuild/transaction_test.go

@@ -2293,7 +2293,7 @@ func TestVerifyChallengeTxSigners_invalidServerAndClientSignersFailsSignerSeed(t
 
  err := tx.Build()
  require.NoError(t, err)
- err = tx.Sign(serverKP, clientKP2, clientKP2)
+ err = tx.Sign(serverKP, clientKP2)
  assert.NoError(t, err)
  tx64, err := tx.Base64()
  require.NoError(t, err)