Remove unused shoulda-matchers

It turns out, we weren't actually using them! Maybe left over from some
past implementation?

Author: stevenharman

Gemfile

@@ -12,4 +12,3 @@ gem 'rspec-rails', '~> 5.0'
 gem 'rubocop', '~> 1.22'
 gem 'rubocop-rails', '~> 2.12'
 gem 'rubocop-rspec', '~> 2.5'
-gem 'shoulda-matchers', '~> 5.0'

spec/spec_helper.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'shoulda-matchers'
-
 RSpec.configure do |config|
   config.filter_run focus: true
   config.run_all_when_everything_filtered = true