Remove explicit subject(:cop) from specs

In rubocop/rubocop#7970 the shared context
`config` was updated to include the following:

    let(:cop) do
      cop_class.new(config, cop_options)
               .tap { |cop| cop.processed_source = processed_source }
    end

This is functionally equivalent to what we have all over our specs:

    subject(:cop) { described_class.new(config) }

The downside is that our specs become a less explicit. Actually they now
have a mystery guest, which may be considered a smell.

spec/rubocop/cop/rspec_rails/http_status_spec.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 
 RSpec.describe RuboCop::Cop::RSpec::Rails::HttpStatus, :config do
-  subject(:cop) { described_class.new(config) }
-
   context 'when EnforcedStyle is `symbolic`' do
     let(:cop_config) { { 'EnforcedStyle' => 'symbolic' } }