Merge #21100: test: remove unused function xor_bytes

f64adc1eedff9d342b49d7e6428b2da21130c23c test: remove unused function xor_bytes (Sebastian Falbesoner)

Pull request description:

  The function `xor_bytes` was introduced in commit 3c226639eb134314a0640d34e4ccb6148dbde22f (#19953, BIP340-342 validation), even [code-reviewed](https://github.com/bitcoin/bitcoin/pull/19953/files#r509383731), but actually never used. The [default signing algorithm in BIP340](https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki#Default_Signing) needs a xor operation, but this step is currently done by a single xor operation on large integer operands:

  ```
  t = (sec ^ int.from_bytes(TaggedHash("BIP0340/aux", aux), 'big')).to_bytes(32, 'big')
  ```

  Alternatively, we could keep the function and as well use it:
  ```diff
  --- a/test/functional/test_framework/key.py
  +++ b/test/functional/test_framework/key.py
  @@ -492,7 +492,7 @@ def sign_schnorr(key, msg, aux=None, flip_p=False, flip_r=False):
       P = SECP256K1.affine(SECP256K1.mul([(SECP256K1_G, sec)]))
       if SECP256K1.has_even_y(P) == flip_p:
           sec = SECP256K1_ORDER - sec
  -    t = (sec ^ int.from_bytes(TaggedHash("BIP0340/aux", aux), 'big')).to_bytes(32, 'big')
  +    t = xor_bytes(sec.to_bytes(32, 'big'), TaggedHash("BIP0340/aux", aux))
       kp = int.from_bytes(TaggedHash("BIP0340/nonce", t + P[0].to_bytes(32, 'big') + msg), 'big') % SECP256K1_ORDER
       assert kp != 0
       R = SECP256K1.affine(SECP256K1.mul([(SECP256K1_G, kp)]))
  ```

ACKs for top commit:
  practicalswift:
    cr ACK f64adc1eedff9d342b49d7e6428b2da21130c23c: untested unused code should be removed

Tree-SHA512: e9afae303488f19c6f6f44874d3537ed1c8164a197490e2b4e34853db886b858825b719648fa1a30b95177dcee9cf241f94ee9b835f0a2cae07024ce38a8c090

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