samples: bpf: remove unnecessary include options in Makefile

Due to recent change of include path at commit b552d33c


("samples/bpf: fix include path in Makefile"), some of the
previous include options became unnecessary.

This commit removes duplicated include options in Makefile.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>