Bug 11820: remove dead code in basket group management

Bug 11820: remove dead code in basket group management

There was some code in acqui/basketgroup.pl that was apparently
intended to let one create a basket group for no (or an unknown)
vendor.  However, this code was never reached, as there is nothing
in the templates that invokes basketgroup.pl with 'add' as the
operation that doesn't also pass the vendor ID along.

This patch removes that dead code.

To test:

[1] Create a new basket group for a vendor and verify that it is
    created correctly.
[2] Edit an existing basket group, including moving baskets in and
    and out of, and verify that it works.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
No regressions found, passes all tests and QA script.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>