wireguard: selftests: remove CONFIG_SPARSEMEM_VMEMMAP=y from qemu kernel config

wireguard: selftests: remove CONFIG_SPARSEMEM_VMEMMAP=y from qemu kernel config

It's no longer user-selectable (and the default was already "y"), so
let's just drop it.

It was never really relevant to the wireguard selftests either way.

Cc: Shuah Khan <shuah@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20250910013644.4153708-4-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>