opkg: remove opkg-build script

This package building script is several years old and unused by buildroot.
opkg users looking for a current version of this script should visit the
opkg-utils repository at http://git.yoctoproject.org/cgit/cgit.cgi/opkg-utils/

,
where this script is maintained and versioned.

Signed-off-by: Vanya Sergeev <vsergeev@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>