IB/core: Get rid of redundant verb ib_destroy_mr

This was added in a thought of uniting all mr allocation
and deallocation routines but the fact is we have a single
deallocation routine already, ib_dereg_mr.

And, move mlx5_ib_destroy_mr specific logic into mlx5_ib_dereg_mr
(includes only signature stuff for now).

And, fixup the only callers (iser/isert) accordingly.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>