drop /run/initramfs/upgrade.conf

Nothing's going to change these paths; let's just simplify things and
not mess around with that file. If we're in upgrade-switch-root.target
and there's a mountpoint at /system-upgrade-root, we're good to go.

(cherry picked from commit ac69fbbe32bc37e998c177ac1e14353dff279cb6)
Remove /run/initramfs/upgrade.conf from the retro init.

Resolves: rhbz#1030561