package/skeleton: remove legacy mkdir

We've had this mkdir, in various forms, for aeons. It dates back to
the inception of Buildroot, at the dawn of ages, when the toolchain
support was still nascent. Evolution had it morph from /include, to
a tuple-prefixed directory and a symlink there-to, then back to the
/include directory, to the final /usr/include directory we've had
for the last 9 years.

Anything that wants to install headers is supposed to be creating
that directory beforehand; we don't need this legacy mkdir now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>