Remove the code for generating binutils-aarch64 etc. packages, as these
live upstream in Alpine now. Having the generator scripts for binutils
cross pkgs is therefore not useful anymore and would just bitrot if we
kept it.

Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20231126121617.3855-1-ollieparanoid@postmarketos.org%3E