Revert "DEV: Avoid duplication of gems in gemfile." (#14784)

This reverts commit 2818ebd19f.

Gemfile

@@ -181,9 +181,14 @@ group :development do
   gem 'yaml-lint'
 end
 
-group ENV["ALLOW_DEV_POPULATE"] == "1" ? :production : :development do
+if ENV["ALLOW_DEV_POPULATE"] == "1"
   gem 'discourse_dev_assets'
   gem 'faker', "~> 2.16"
+else
+  group :development do
+    gem 'discourse_dev_assets'
+    gem 'faker', "~> 2.16"
+  end
 end
 
 # this is an optional gem, it provides a high performance replacement