sys-kernel/linux-headers: drop 2.4 linux-headers

sys-kernel/linux-headers: drop 2.4 linux-headers

In June 2016 commit 81c226e451be564a545696f93fc5880ebc160812
broke support for kernel-version based ARCH detection for
    i386, x86_64, ppc and powerpc64.

Not that many arches are left supporting these headers.
All of them should work fine on hewer kernels and toolchains.

Broken-by: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=81c226e451be564a545696f93fc5880ebc160812
Closes: https://bugs.gentoo.org/653852
Package-Manager: Portage-2.3.31, Repoman-2.3.9