Fix minor interface bug: No-SU prompt after no-OS prompt

When /system is wiped in TWRP and afterwards Reboot -> Recovery is selected the "No OS is installed, are you sure you want to reboot?" prompt appears. After swiping to "Reboot anyway" the "Current ROM is not rooted, do you wish to install SuperSU?" prompt appears.
This change removes "Do you wish to install SuperSU" prompt after "No OS installed" prompt.

Change-Id: Ie1157afdf5cf1cb81d6647269172d6e63af9f4b0