selftests: hid: enforce new attach API

Now that the new API for hid_bpf_attach_prog() is in place, ensure we
get an fd when calling this function. And remove the fallback code.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>