Remove deprecated Hash#diff with no replacement.

If you're using it to compare hashes for the purpose of testing,
please use MiniTest's assert_equal instead.

activesupport/CHANGELOG.md

+*   Remove deprecated `Hash#diff` with no replacement.
+
+    If you're using it to compare hashes for the purpose of testing, please use
+    MiniTest's `assert_equal` instead.
+
+    *Carlos Antonio da Silva*
+
 *   Remove deprecated `Date#to_time_in_current_zone` in favour of `Date#in_time_zone`.
 
     *Vipul A M*

activesupport/lib/active_support/core_ext/hash.rb

 require 'active_support/core_ext/hash/conversions'
 require 'active_support/core_ext/hash/deep_merge'
-require 'active_support/core_ext/hash/diff'
 require 'active_support/core_ext/hash/except'
 require 'active_support/core_ext/hash/indifferent_access'
 require 'active_support/core_ext/hash/keys'

activesupport/lib/active_support/core_ext/hash/diff.rb

-require 'active_support/deprecation'
-
-class Hash
-  # Returns a hash that represents the difference between two hashes.
-  #
-  #   {1 => 2}.diff(1 => 2)         # => {}
-  #   {1 => 2}.diff(1 => 3)         # => {1 => 2}
-  #   {}.diff(1 => 2)               # => {1 => 2}
-  #   {1 => 2, 3 => 4}.diff(1 => 2) # => {3 => 4}
-  def diff(other)
-    ActiveSupport::Deprecation.warn "Hash#diff is no longer used inside of Rails, and is being deprecated with no replacement. If you're using it to compare hashes for the purpose of testing, please use MiniTest's assert_equal instead."
-    dup.
-      delete_if { |k, v| other[k] == v }.
-      merge!(other.dup.delete_if { |k, v| has_key?(k) })
-  end
-end

activesupport/test/core_ext/hash_ext_test.rb

@@ -701,12 +701,6 @@ def test_reverse_merge
     assert_equal expected, merged
   end
 
-  def test_diff
-    assert_deprecated do
-      assert_equal({ :a => 2 }, { :a => 2, :b => 5 }.diff({ :a => 1, :b => 5 }))
-    end
-  end
-
   def test_slice
     original = { :a => 'x', :b => 'y', :c => 10 }
     expected = { :a => 'x', :b => 'y' }