Gemfile: remove duplicate ffi dependency

We already have it in them gemspec, we don't need it in the Gemfile as
well.

Author: bastelfreak

Gemfile

@@ -13,8 +13,3 @@ gem 'packaging', require: false
 
 local_gemfile = File.expand_path('Gemfile.local', __dir__)
 eval_gemfile(local_gemfile) if File.exist?(local_gemfile)
-
-group(:integration, optional: true) do
-  # 1.16.0 - 1.16.2 are broken on Windows
-  gem 'ffi', '>= 1.15.5', '< 1.17.0', '!= 1.16.0', '!= 1.16.1', '!= 1.16.2', require: false
-end