[ruby/pp] Remove patch added for Ruby 2.6/JRuby 9.3

https://github.com/ruby/pp/commit/09dae96129

test/test_pp.rb

@@ -5,15 +5,6 @@ require 'delegate'
 require 'test/unit'
 require 'ruby2_keywords'
 
-# Define bind_call for Ruby 2.6 and earlier, to allow testing on JRuby 9.3
-class UnboundMethod
-  unless public_method_defined?(:bind_call)
-    def bind_call(obj, *args, &block)
-      bind(obj).call(*args, &block)
-    end
-  end
-end
-
 module PPTestModule
 
 class PPTest < Test::Unit::TestCase