test: remove unused CHANGE_{XPRV,XPUB} constants

These constants exist since the introduction of the functional test
wallet_taproot.py (2667366aaa), but they
have never been used.

test/functional/wallet_taproot.py

@@ -156,9 +156,6 @@ KEYS = [
     }
 ]
 
-CHANGE_XPRV = "tprv8ZgxMBicQKsPcyDrWwiecVnTtFmfRwbfFqEfR4ZGWvq5aTTwLBWmAm5zrbMcYtb9gQNFfhRfqhhrBG37U3nhmXxEgeEPBJGHAPrHCrAd1WX"
-CHANGE_XPUB = "tpubD6NzVbkrYhZ4WSFeQbPF1uSaTHHbbGnZq8qShabZwCdUQwihxaLMMFhs2kidGF2qrRKiQVqw8VoyuTHj1bZqmMXMeciaU1gBjWA1sim2zUB"
-
 
 def key(hex_key):
     """Construct an x-only pubkey from its hex representation."""