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/messages.py

@@ -30,7 +30,7 @@ import struct
 import time
 
 from test_framework.siphash import siphash256
-from test_framework.util import hex_str_to_bytes, assert_equal
+from test_framework.util import assert_equal
 
 MAX_LOCATOR_SZ = 101
 MAX_BLOCK_BASE_SIZE = 1000000
@@ -197,7 +197,7 @@ def from_hex(obj, hex_string):
     Note that there is no complementary helper like e.g. `to_hex` for the
     inverse operation. To serialize a message object to a hex string, simply
     use obj.serialize().hex()"""
-    obj.deserialize(BytesIO(hex_str_to_bytes(hex_string)))
+    obj.deserialize(BytesIO(bytes.fromhex(hex_string)))
     return obj