Remove i18n_spec file (#2014)

These were being skipped with `xit`, but we also just added the `unused` task to
the CI server, and it will be caught there. That seems good enough for this,
instead of requiring a full keys check on every local spec suite run.

spec/i18n_spec.rb

-# frozen_string_literal: true
-require 'i18n/tasks'
-
-RSpec.describe 'I18n' do
-  let(:i18n)         { I18n::Tasks::BaseTask.new }
-  let(:missing_keys) { i18n.missing_keys }
-  let(:unused_keys)  { i18n.unused_keys }
-
-  xit 'does not have missing keys' do
-    expect(missing_keys).to be_empty, "Missing #{missing_keys.leaves.count} i18n keys, run `i18n-tasks missing' to show them"
-  end
-
-  xit 'does not have unused keys' do
-    expect(unused_keys).to be_empty, "#{unused_keys.leaves.count} unused i18n keys, run `i18n-tasks unused' to show them"
-  end
-end