test: Remove unused scheduler.h include from the common setup

Summary:
The common setup is included in virtually all tests, so it should be
as slim as possible.

This is a backport of core#19090 [2/7]
https://github.com/bitcoin/bitcoin/pull/19090/commits/fa9819695aac260be0ba170eb15ecba8cb519843

Test Plan: ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9956