test_runner: Remove unused --force option

Summary:

When someone calls the script they already have all intention to call it, no need to specify a redundant --force.

This is a backport of Core PR15370

Test Plan: ninja && ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8357