Remove freebsd build instructions

Summary:
Remove build instructions for freebsd and references to these instruction, as suggested by deadalnix
(https://reviews.bitcoinabc.org/D7472?id=23549#inline-42917).

Test Plan:

bundle exec jekyll serve

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7474