DEV: remove foreman gem and unsupported Procfile

Launching Discourse no longer should require foreman in dev. We can simply
use `bin/unicorn` which automatically launches a sidekiq worker.

The foreman gem depends on thor ~> 0.19.1 which is no longer supported in
rails 6. So this pre-emptively prepares us for it.

Gemfile

@@ -143,7 +143,6 @@ group :development do
   gem 'better_errors'
   gem 'binding_of_caller'
   gem 'annotate'
-  gem 'foreman', require: false
 end
 
 # this is an optional gem, it provides a high performance replacement