activesupport: Delete a dummy definition: Object#sum (#450)

It seems this definition was added to avoid the trouble on `rbs
prototype rb` and refinements.  But it was gone away now because
`rbs prototype rb` command ignores refinements now.

refs: ruby/rbs#487

gems/activesupport/6.0/patch.rbs

@@ -1,9 +1,3 @@
-class Object
-  # Object#sum is actually not defined, but rbs prototype rb generates it mistakenly due to `class << Benchmark`.
-  # So we need to define it to suppress the alias error.
-  def sum: () -> untyped
-end
-
 module ActiveSupport
   class TestCase < ::Minitest::Test
     # It is necessary to satisfy alias target.