qa/suites/krbd: use a standard fixed-1 cluster in unmap subsuite

qa/suites/krbd: use a standard fixed-1 cluster in unmap subsuite

A custom "fixed-1, but with the client on a separate node" cluster was
needed only for pre-single-major.yaml kernel which is no longer around.
This can be a single-node job now -- see commits 311a450163cf
("krbd/unmap: put client.0 on a separate remote") and 39a579144cd8
("qa/suites/krbd: drop pre-single-major test").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 9bfd67974462f4c53610d286862a046d8cb07984)