Add cms_verify=y to archiso_pxe-linux.cfg

/configs/releng/syslinux/archiso_pxe-linux.cfg has checksum and verify, but not cms_verify=y.

Since cms_verify=y has been confirmed to work in archlinux-2022.10.01-x86_64.iso, the new cms_verify=y can now be added.

On the same note, checksum and verify can now be removed, since they essentially serve the same purpose and performing all the checks just needlessly delays boot.