[website] Remove the avalanche post-consensus activation banner

Summary:
This has been sitting there for 2 years, it's no longer anything new.
We keep the banner file so it can be reused for pre-consensus when time has come.

Test Plan:

bundle exec jekyll serve --incremental

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16850