samples/bpf: Remove duplicate option from xdpsock

The '-f' option is shown twice in the usage(). This patch removes the
outdated version.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191114162847.221770-1-andre.guedes@intel.com