Remove useless `feature: true` / `:feature` RSpec tags

We have a lot of feature: true:

› git grep "feature: true" spec/ | wc -l
     254

› git grep " :feature" spec/ | wc -l
      59

These are actually useless. What is useful is type: :feature, but we don't have to set it explicitly for specs under spec/features: https://github.com/rspec/rspec-rails#feature-specs