qa/suites/krbd: drop pre-single-major test

qa/suites/krbd: drop pre-single-major test

Single-major mapping scheme was introduced in 2014 and became the
default in 2017.  It's getting increasingly difficult to build and,
more importantly, to boot a 10 year old kernel with recent userspace
(systemd, etc).  If someone is still running such a kernel, it's
really unlikely that they would have the most recent rbd CLI tool
installed.

Fixes: https://tracker.ceph.com/issues/51845
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 39a579144cd8b97dd338bfcf36c06ad4bcaed526)