Make puppet-syntax into an implicit dependency

The most recent versions of puppet-syntax depend on a version of rake
that's not set as compatible with other gems that we're using (at least
voxpupuli-test even in the most recent version of that gem).

puppet-syntax is already being pulled in indirectly as a dependency of
puppetlabs_spec_helper so we can stop explicitly specifying it in our
Gemfile

Gemfile

@@ -14,7 +14,6 @@ group :tests do
  gem 'puppet_metadata', '~> 4.0', require: false if Gem::Version.new(RUBY_VERSION.dup) >= Gem::Version.new('2.7.0')
  gem 'puppet_metadata', '~> 2.1', require: false if Gem::Version.new(RUBY_VERSION.dup) < Gem::Version.new('2.7.0')
  # rubocop:enable Bundler/DuplicatedGem
- gem 'puppet-syntax', '~> 3.3'
  gem 'rspec-puppet', '>= 2.4.0'
  # This draws in rubocop and other useful gems for puppet tests
  # rubocop:disable Bundler/DuplicatedGem