remove unnecessary require 'core_ext/numeric/time'.

Previously it depended on `core_ext/numeric/time`, but now it doesn't, so we removed require.

Previous code

``` ruby
class Integer
  def months
    ActiveSupport::Duration.new(self * 30.days, [[:months, self]])
  end
  alias :month :months
  def years
    ActiveSupport::Duration.new(self * 365.25.days, [[:years, self]])
  end
  alias :year :years
end
```

Now the code

``` ruby
class Integer
  def months
    ActiveSupport::Duration.months(self)
  end
  alias :month :months

  def years
    ActiveSupport::Duration.years(self)
  end
  alias :year :years
end
```

activesupport/lib/active_support/core_ext/integer/time.rb

 # frozen_string_literal: true
 
 require "active_support/duration"
-require "active_support/core_ext/numeric/time"
 
 class Integer
   # Returns a Duration instance matching the number of months provided.