test: Remove True argument to CBlock::serialize

Unnamed arguments are confusing as to what they mean without looking up
the function signature.

Since segwit is active by default in regtest, and all blocks are
serialized with witness (#15664, c459c5f), remove the argument
`with_witness=True` from all calls to `CBlock::serialize` and
`BlockTransactions::serialize`.

This diff has been created with a script, but is better reviewed without
a scripted diff.

sed -i --regexp-extended -e 's/block(_?[2a-z]*)\.serialize\([a-z_]*=?True/block\1.serialize(/g' $(git grep -l serialize ./test)

test/functional/feature_nulldummy.py

@@ -108,7 +108,7 @@ class NULLDUMMYTest(BitcoinTestFramework):
         witness and add_witness_commitment(block)
         block.rehash()
         block.solve()
-        node.submitblock(block.serialize(True).hex())
+        node.submitblock(block.serialize().hex())
         if (accept):
             assert_equal(node.getbestblockhash(), block.hash)
             self.tip = block.sha256