RDMA/nes: Remove useless usecnt variable and redundant memset

The internal design of RDMA/core ensures that there dealloc ucontext will
be called only if alloc_ucontext succeeded, hence there is no need to
manage internal variable to mark validity of ucontext.

As part of this change, remove redundant memeset too.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>