Remove :time_travel RSpec metadata tag as it results in flaky tests

- Also, while we're at it, let's make the previous `:time_travel`
  use-case test more descriptive about what it is testing

ee/spec/models/gitlab_subscription_spec.rb

@@ -9,8 +9,8 @@
     let_it_be(plan) { create(plan) }
   end
 
-  describe 'default values', time_travel: 30.days do
-    specify do
+  describe 'default values', :freeze_time do
+    it 'defaults start_date to the current date' do
       expect(subject.start_date).to eq(Date.today)
     end
   end

spec/support/time_travel.rb

@@ -9,14 +9,6 @@
     freeze_time { example.run }
   end
 
-  config.around(:example, :time_travel) do |example|
-    duration = example.metadata[:time_travel]
-
-    raise 'The time_travel RSpec metadata must have an ActiveSupport::Duration value (such as `30.days`).' unless duration.is_a?(ActiveSupport::Duration)
-
-    travel(duration) { example.run }
-  end
-
   config.around(:example, :time_travel_to) do |example|
     date_or_time = example.metadata[:time_travel_to]
 

spec/support_specs/time_travel_spec.rb

@@ -9,18 +9,6 @@
     end
   end
 
-  describe ':time_travel' do
-    today = Date.current
-
-    it 'time-travels by the given duration', time_travel: 3.days do
-      expect(Date.current).to eq(today + 3.days)
-    end
-
-    it 'works with negative durations', time_travel: -5.days do
-      expect(Date.current).to eq(today - 5.days)
-    end
-  end
-
   describe ':time_travel_to' do
     it 'time-travels to the specified date', time_travel_to: '2020-01-01' do
       expect(Date.current).to eq(Date.new(2020, 1, 1))