[test] remove unnecessary transaction rehashing

Summary: The create_coinbase function already calls CTransaction.calc_sha256() immediately prior to returning the transaction, so it is not necessary to call CTransaction.rehash() if the transaction is not modified.

Test Plan: test/functional/test_runner.py abc*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11319