Remove enable_mocktime in rpc_listtransactions

Summary:
Mocktime is unused in the test.
Depends on D2628

Test Plan:

./test/functional/test_runner.py rpc_listtransactions

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2629