Fixed mempool test, block size was not needed to be set to test the mempool

Summary: Removed LEGACY_MAX_BLOCK_SIZE, so the mempool test is independent of the max block size

Test Plan: ../qa/puller-tester/rpc-tests.py mempool-accept-txn.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D244