compressdev: clarify when private xform can be cleared

As the private_xform data can be shared by many operations
and across queue_pairs, it would be performance impacting
for PMDs to track inflights associated with one. It makes
more sense to push the responsibility to the application to
keep track of its usage and only delete the private_xform when
there are no more ops using it.

Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>