Remove unnecessary --force-yes from installation script

Summary:
--force-yes is not recommended (see https://lists.debian.org/debian-devel/2015/06/msg00145.html ), and we download the proper keys,
so it's not clear why this is needed.

Test Plan: CI should build and pass tests without issue

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5835