Drop knapsack

It constantly fails with "Mysql2::Error: This connection is in use by...".
Might be because of database cleaner? But until we can get this stable,
rip it out again.

Replaced by a matrix based on test type

Gemfile

@@ -275,8 +275,6 @@ group :test do
   gem 'rails-controller-testing'
   # For managing the environment
   gem 'climate_control'
-  # parallel tests
-  gem 'knapsack'
 end
 
 group :development, :test do