test: avoid unneeded hash -> uint256 -> hash roundtrips

In the functional test framework, we often treat hashes
as uint256 integers, which seems to be confusing and for no
good reason, as hashes are just sequences of bytes. This commit
gets rid of obvious internal instances of that where individual
functional tests are not affected. In the long-term, it might make
sense to store other hashes (mostly txids) as actual bytes to
avoid annoying conversions and improve code readability.

test/functional/test_framework/blocktools.py

@@ -28,7 +28,6 @@ from .messages import (
     ser_uint256,
     tx_from_hex,
     uint256_from_compact,
-    uint256_from_str,
     WITNESS_SCALE_FACTOR,
 )
 from .script import (
@@ -111,8 +110,8 @@ def create_block(hashprev=None, coinbase=None, ntime=None, *, version=None, tmpl
     return block
 
 def get_witness_script(witness_root, witness_nonce):
-    witness_commitment = uint256_from_str(hash256(ser_uint256(witness_root) + ser_uint256(witness_nonce)))
-    output_data = WITNESS_COMMITMENT_HEADER + ser_uint256(witness_commitment)
+    witness_commitment = hash256(ser_uint256(witness_root) + ser_uint256(witness_nonce))
+    output_data = WITNESS_COMMITMENT_HEADER + witness_commitment
     return CScript([OP_RETURN, output_data])
 
 def add_witness_commitment(block, nonce=0):