libelf: Removes the package

The elfutils package provides a more recent version of the
libelf. Some packages such as ltrace need this more recent
version. Having two packages providing the same library leads to some
conflicts or dependency problems. For instance at the end we had only
one libelf.a when the 2 packages were selected.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>