Remove redundant spec

* See #1319 (comment)
  for details

Author: eregon

core/enumerable/to_set_spec.rb

@@ -27,12 +27,4 @@
       set.to_a.sort.should == [1, 2, 3]
     end
   end
-
-  it "does not need explicit `require 'set'`" do
-    output = ruby_exe(<<~RUBY, options: '--disable-gems', args: '2>&1')
-      puts [1, 2, 3].to_set.to_a.inspect
-    RUBY
-
-    output.chomp.should == "[1, 2, 3]"
-  end
 end