test: remove unused function xor_bytes #21100

The function xor_bytes was introduced in commit 3c226639eb134314a0640d34e4ccb6148dbde22f (#19953, BIP340-342 validation), even code-reviewed, but actually never used. The default signing algorithm in BIP340 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:

--- 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)]))

According to vulture there is more unused code in the same file:

bitcoin/test/functional/test_framework/key.py:27: unused function 'xor_bytes' (60% confidence)
bitcoin/test/functional/test_framework/key.py:277: unused method 'verify_ecdsa' (60% confidence)
bitcoin/test/functional/test_framework/key.py:517: unused variable 'sign_pubkey' (60% confidence)

@fanquake: Thanks for pointing out. verify_ecdsa is indeed unused, but was in the dead-code linters whitelist since its introduction (removed recently in https://github.com/bitcoin/bitcoin/pull/16961/commits/f4beb4996d27f2cdaf4f0a63e7dc044bf17decce), so I guess the community's wish is to keep it (to have a complete Python ECDSA implementation)? Force-pushed now with the change of replacing sign_pubkey with an underscore.

cr ACK f64adc1eedff9d342b49d7e6428b2da21130c23c: untested unused code should be removed