Revert "Add CBMC package build file for Arch Linux"

This reverts commit 735c152.

Last time this file was maintained was 4 years ago. It no
longer makes sense for us to keep it around as no one maintains
it, and for anyone that wants to use CBMC under Arch Linux,
there are packages in AUR that are better maintained than this
one.

Author: NlightNFotis