app/crypto-perf: do not populate the mbufs at init

For throughput and latency tests, it is not required
to populate the mbufs with any test vector.
For verify test, there is already a function that rewrites
the mbufs every time they are going to be used with
crypto operations.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>