iproute2: revert add libcap support, enabled in ip-full

iproute2: revert add libcap support, enabled in ip-full

This reverts commit a6da3f9ef746101b84a6f530f5a40de28341b69a.

The libcap isn't as optional as the commit messages suggests. A hard
dependency to the libcap package is added, which is only available in
the external packages feed. Therefore it is impossible to package
ip-full without having the external packages feed up and running, which
is a regression to the former behaviour.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>