Remove enable_mocktime from wallet_listreceivedby

Summary:
The enable_mocktime() call has been resurrected in D2465.
However this line is not necessary and should have been removed with
D1983 because it is part of core PR11055.

Test Plan:

./test/functional/test_runner.py wallet_listreceivedby

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2635