Remove pry_byebug plugin, Padrino now uses Pry if it's present

[Quintasan]

Since padrino/padrino-framework@2a2600a7f9f594aeb3ae8bec3da04ab80d64bbf0 Padrino uses Pry by default if it's present. That commit renders pry_byebug plugin less useful since now you only have to add pry_byebug to your Gemfile.