xsk: remove unnecessary assignment

Since xdp_umem_query() was added one assignment of bpf.command was
missed from cleanup. Removing the assignment statement.

Fixes: 84c6b868 ("xsk: don't allow umem replace at stack level")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>