remove JSON support from homebrew-fork

and vendor/okjson.rb in our own codebase, recently adopted
from homebrew-fork.  JSON reads are used within Homebrew
to support download mirrors and GitHub interface: two things
we might wish to support in the future, but do not at this
time.
Merge pull request #8448 from rolandwalker/remove_homebrew_json

remove JSON support from homebrew-fork