Undeprecate plural positional argument

```ruby
pluralize people.count, 'person', 'people'
```
reads more naturally than
```ruby
pluralize people.count, 'person', plural: 'people'
```
so let's not deprecate it.

We could label both, but that's a mouthful:
```ruby
pluralize people.count, singular: 'person', plural: 'people'
```

(The `plural:` kwarg shipped in 5.0.0, so we're keeping it.)

actionview/lib/action_view/helpers/text_helper.rb

@@ -225,14 +225,7 @@ def excerpt(text, phrase, options = {})
       #
       #   pluralize(2, 'Person', locale: :de)
       #   # => 2 Personen
-      def pluralize(count, singular, deprecated_plural = nil, plural: nil, locale: I18n.locale)
-        if deprecated_plural
-          ActiveSupport::Deprecation.warn("Passing plural as a positional argument " \
-            "is deprecated and will be removed in Rails 5.1. Use e.g. " \
-            "pluralize(1, 'person', plural: 'people') instead.")
-          plural ||= deprecated_plural
-        end
-
+      def pluralize(count, singular, plural_arg = nil, plural: plural_arg, locale: I18n.locale)
         word = if (count == 1 || count =~ /^1(\.0+)?$/)
           singular
         else

actionview/test/template/text_helper_test.rb

@@ -379,6 +379,8 @@ def test_pluralization
     assert_equal("1.25 counts", pluralize("1.25", "count"))
     assert_equal("1.0 count", pluralize("1.0", "count"))
     assert_equal("1.00 count", pluralize("1.00", "count"))
+    assert_equal("2 counters", pluralize(2, "count", "counters"))
+    assert_equal("0 counters", pluralize(nil, "count", "counters"))
     assert_equal("2 counters", pluralize(2, "count", plural: "counters"))
     assert_equal("0 counters", pluralize(nil, "count", plural: "counters"))
     assert_equal("2 people", pluralize(2, "person"))
@@ -405,12 +407,6 @@ def test_localized_pluralization
     end
   end
 
-  def test_deprecated_plural_as_positional_argument
-    assert_deprecated do
-      pluralize(2, "count", "counters")
-    end
-  end
-
   def test_cycle_class
     value = Cycle.new("one", 2, "3")
     assert_equal("one", value.to_s)