doc: remove incorrect limitation on AESNI-MB PMD

AESNI MB PMD supports sessionless operations,
but the documentation was stating that only
operations with session were supported.

Fixes: 924e84f87306 ("aesni_mb: add driver for multi buffer based crypto")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>