Remove Eoan from PPA releases

Summary:
Eoan is EOL and there's a small additional maintenance burden to
supporting it, so let's drop it.

Test Plan:

./debian-packages.sh -p jasonbcox-bitcoin-abc jasonbcox

See result here: https://launchpad.net/~jasonbcox/+archive/ubuntu/bitcoin-abc/+packages

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7186