[logging] Don't log REJECT code when transaction is rejected

Remove the BIP61 REJECT code from error messages and logs when a
transaction is rejected.

BIP61 support was removed from Bitcoin Core in
fa25f43ac5. The REJECT codes will be
removed from the codebase entirely in the following commit.

test/functional/feature_cltv.py

@@ -126,7 +126,7 @@ class BIP65Test(BitcoinTestFramework):
         # First we show that this tx is valid except for CLTV by getting it
         # rejected from the mempool for exactly that reason.
         assert_equal(
-            [{'txid': spendtx.hash, 'allowed': False, 'reject-reason': '64: non-mandatory-script-verify-flag (Negative locktime)'}],
+            [{'txid': spendtx.hash, 'allowed': False, 'reject-reason': 'non-mandatory-script-verify-flag (Negative locktime)'}],
             self.nodes[0].testmempoolaccept(rawtxs=[spendtx.serialize().hex()], maxfeerate=0)
         )