Avoid raising a NameError on FreeBSD using Date

Hello

This is just a little pull request to address #11723. The `Date` object has a `xmlschema` method starting with Ruby 1.9 so we were assuming that we could safely remove this method and redefine it later but the call to remove_method throws a `NameError` on FreeBSD so we should rely on `remove_possible_method` instead.

This call is actually needed to avoid warnings when running the test suite.

Have a nice day.