net: bpf: remove XDP_QUERY_XSK_UMEM enumerator

Commit c9b47cc1 ("xsk: fix bug when trying to use both copy and
zero-copy on one queue id") moved the umem query code to the AF_XDP
core, and therefore removed the need to query the netdevice for a
umem.

This patch removes XDP_QUERY_XSK_UMEM and all code that implement that
behavior, which is just dead code.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>