Avoid raising a NameError on FreeBSD using Date

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.

Fixes #11723
Merge pull request #13561 from robin850/issue-11723

Avoid raising a NameError on FreeBSD using Date