Remove DatabaseCleaner

This should JustWork™ in Rails 5.1+. I’m tracking
rspec/rspec-rails#1813 so
We can eventually integrate system specs (the now
“official” Rails way) instead of feature specs.

Author: citizen428

Gemfile

@@ -111,7 +111,6 @@ end
 group :test do
   gem 'bundler-audit', require: false
   gem 'capybara', '~> 2.13.0'
-  gem 'database_cleaner'
   gem 'factory_girl_rails'
   gem 'poltergeist'
   gem 'rspec-rails', '~> 3.5'

Gemfile.lock

@@ -111,7 +111,6 @@ GEM
     connection_pool (2.2.1)
     crack (0.4.3)
       safe_yaml (~> 1.0.0)
-    database_cleaner (1.5.3)
     diff-lcs (1.3)
     erubi (1.6.0)
     execjs (2.7.0)
@@ -338,7 +337,6 @@ DEPENDENCIES
   bundler-audit
   byebug
   capybara (~> 2.13.0)
-  database_cleaner
   devise!
   draper!
   factory_girl_rails