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

Gemfile.lock

@@ -144,8 +144,6 @@ GEM
     fastimage (2.1.3)
     ffi (1.9.25)
     flamegraph (0.9.5)
-    foreman (0.85.0)
-      thor (~> 0.19.1)
     fspath (3.1.0)
     gc_tracer (1.5.1)
     git (1.5.0)
@@ -478,7 +476,6 @@ DEPENDENCIES
   fast_xs
   fastimage
   flamegraph
-  foreman
   gc_tracer
   highline (~> 1.7.0)
   hiredis

Procfile

@@ -1,2 +0,0 @@
-web: bundle exec rails server -p $PORT
-worker: bundle exec sidekiq -e $RAILS_ENV