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

INSTALL.md

@@ -3,7 +3,6 @@ Building Bitcoin
 
 See platform-specific instructions on building the various
 elements of the Bitcoin ABC implementation of Bitcoin:
-- [freebsd](doc/build-freebsd.md)
 - [osx](doc/build-osx.md)
 - [unix](doc/build-unix.md)
 - [windows](doc/build-windows.md)

doc/build-unix.md

@@ -2,8 +2,6 @@ UNIX BUILD NOTES
 ====================
 Some notes on how to build Bitcoin ABC in Unix.
 
-(For FreeBSD specific instructions, see `build-freebsd.md` in this directory.)
-
 To Build
 ---------------------