Remove unused argument to ThreadHTTP(...)
Summary:
Partial backport of Core PR13554 (the other commits do not apply to ABC)
https://github.com/bitcoin/bitcoin/pull/13554/commits/05dbb0c042ba647bf83c15f059c41620e2164337
Completes T538
Test Plan: ninja check
Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2587