Revert "Added check for variant stockrecord"

This reverts commit a50ed82db4dff54d60ecc489c2a769badcb92d80.

It was committed because has_stockrecord was buggy and made this
workaround necessary. It does not belong on master though, as
has_stockrecord is fixed.
Furthermore, it introduces two bugs:
- Accessing .stockrecord might raise DoesNotExist exceptions
- a product will falsely be shown as not available if it doesn't
  have a stock record, even if it's product class is set to not
  track stock