test: refactor: remove hex_str_to_bytes helper

Use the built-in class method bytes.fromhex() instead,
which is available since Python 3.0.

test/functional/test_framework/util.py

@@ -5,7 +5,6 @@
 """Helpful routines for regression testing."""
 
 from base64 import b64encode
-from binascii import unhexlify
 from decimal import Decimal, ROUND_DOWN
 from subprocess import CalledProcessError
 import hashlib
@@ -214,10 +213,6 @@ def count_bytes(hex_string):
     return len(bytearray.fromhex(hex_string))
 
 
-def hex_str_to_bytes(hex_str):
-    return unhexlify(hex_str.encode('ascii'))
-
-
 def str_to_b64str(string):
     return b64encode(string.encode('utf-8')).decode('ascii')
 
@@ -517,7 +512,7 @@ def gen_return_txouts():
     from .messages import CTxOut
     txout = CTxOut()
     txout.nValue = 0
-    txout.scriptPubKey = hex_str_to_bytes(script_pubkey)
+    txout.scriptPubKey = bytes.fromhex(script_pubkey)
     for _ in range(128):
         txouts.append(txout)
     return txouts