[gitian] Remove gitian

Summary: We have been using GUIX for the release builds since a few months already, it's time to remove Gitian completely.

Test Plan:

git grep gitian

Check the only matches are located in the archived release notes or the README for checking older versions signatures.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16127