package/rp-pppoe: drop first patch

This patch has been added to buildroot more than eleven years ago in commit
e9ae739c however it seems that rp-pppoe
builds perfectly fine without it nowadays.

- ac_cv_linux_kernel_pppoe=no just gives a warning;
- HAVE_LINUX_KERNEL_PPPOE is set by configure based on the presence of
  the appropriate header.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>