Remove deprecated escape_key

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -181,14 +181,6 @@ def normalize_key(key, options)
           key
         end
 
-        def escape_key(key)
-          ActiveSupport::Deprecation.warn(<<-MESSAGE.strip_heredoc)
-            `escape_key` is deprecated and will be removed from Rails 5.1.
-            Please use `normalize_key` which will return a fully resolved key or nothing.
-          MESSAGE
-          key
-        end
-
         def deserialize_entry(raw_value)
           if raw_value
             entry = Marshal.load(raw_value) rescue raw_value

activesupport/test/caching_test.rb

@@ -1092,12 +1092,6 @@ def test_read_should_return_a_different_object_id_each_time_it_is_called
     value << "bingo"
     assert_not_equal value, @cache.read("foo")
   end
-
-  def test_can_call_deprecated_escape_key
-    assert_deprecated "`escape_key` is deprecated" do
-      assert_equal 111, @cache.send(:escape_key, 111)
-    end
-  end
 end
 
 class NullStoreTest < ActiveSupport::TestCase