Remove minimum coverage

This clashes with Codecov coverage uploading. If the tests fail chances
are that they didn't hit the threshold and thus, won't be uploaded to
Codecov.

.simplecov

@@ -1,8 +1,6 @@
 #!/bin/env ruby
 
 SimpleCov.start 'rails' do
-  minimum_coverage 54
-
   add_filter '/bin/'
   add_filter '/config/'
   add_filter '/jobs/application_job.rb'