main/mkinitfs: remove code which deletes old initramfs files

Presently, mkinitfs always only generates a single initramfs file under /boot/initramfs-$flavor. A long time ago, the trigger would generated /boot/initramfs-$abi_release instead, thus requiring removal of older initramfs files (see #660 (closed)). Since this is no longer the case the code is no longer needed and can be removed.