BUG Remove caching of statements due to risk of instability

This would cause segfaults in rare situations where statements are reused

I feel that since there's no reliable way to prevent these at this stage (and since this is blocking the merging of CMS) then its prudent to disable this optimisation until a stable solution can be found.

This will allow the test cases at https://github.com/silverstripe/silverstripe-cms/pull/311 to pass again.