[CI] Remove PPA package for Ubuntu 20.04 Focal

Summary: It doesn't provide the required cmake version (3.18) and it's already 2 LTS versions old. Users can still get the release from the official website that will run just fine, or build their own.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16985