mempool: remove callback to register memory area

mempool: remove callback to register memory area

The callback is not required any more since there is a new callback
to populate objects using provided memory area which provides
the same information.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>