Change from rake Task to a single spec

This has the advantage that if you run the whole suite it gets run
automatically but if you run a single test it doesn't. This seems
like the best of both worlds..

lib/tasks/factorybot.rake

@@ -1,8 +0,0 @@
-# frozen_string_literal: true
-
-namespace :factorybot do
-  desc 'Lint FactoryBot factories'
-  task lint: :environment do
-    FactoryBot.lint
-  end
-end

spec/factorybot_lint_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'FactoryBot' do
+  it 'has valid factories' do
+    FactoryBot.lint
+  end
+end