remove_possible_method -> remove_method

  Date#to_time and Date#xmlschema methods defined in Ruby 1.9.3
Merge pull request #8896 from NARKOZ/patch-5

remove_possible_method -> remove_method
Fix regression introduced in pull request 8812

See https://github.com/rails/rails/pull/8812#commitcomment-2416514
Merge pull request #8907 from rubys/master

Fix regression introduced in pull 8812