Gemfile: Remove unneeded facter entry

This was maybe used in the past for CI stuff, but we don't set the
environment variable anywhere.

Author: bastelfreak

Gemfile

@@ -4,8 +4,6 @@ source ENV['GEM_SOURCE'] || 'https://rubygems.org'
 
 gemspec
 
-gem 'facter', ENV.fetch('FACTER_GEM_VERSION', nil), require: false
-
 group :release, optional: true do
   gem 'faraday-retry', '~> 2.1', require: false
   gem 'github_changelog_generator', '~> 1.16.4', require: false