Remove voxpupuli-rubocop and github_changelog_generator dev task blocks.
Developer linting is not our concern when testing the package for compatibility.

--- a/Rakefile
+++ b/Rakefile
@@ -58,23 +58,3 @@ end
 task update: %i[update_common_tests update_meta_schemas]
 
-begin
-  require 'voxpupuli/rubocop/rake'
-rescue LoadError
-  # the voxpupuli-rubocop gem is optional
-end
-
 task default: :test
-
-begin
-  require 'rubygems'
-  require 'github_changelog_generator/task'
-rescue LoadError
-else
-  GitHubChangelogGenerator::RakeTask.new :changelog do |config|
-    config.exclude_labels = %w{duplicate question invalid wontfix wont-fix skip-changelog dependencies}
-    config.user = 'voxpupuli'
-    config.project = 'json-schema'
-    gem_version = Gem::Specification.load("#{config.project}.gemspec").version
-    config.future_release = "v#{gem_version}"
-  end
-end