Remove jh/Gemfile, use Gemfile.jh instead

What does this MR do and why?

Per the discussion, we would like make jh/Gemfile static by getting rid of instance_eval.

1. Rename jh/Gemfile to jh/Gemfile.jh and remove

instance_eval gemfile.gsub('vendor/gems', '../vendor/gems')

2. Add a new function in jh/bin

build_static_jh_gemfile can be utilised by upstream, code-sync and local developers to generate a static jh/Gemfile