Remove the unused 'b' slot

Although we can't opt-out A/B (because bootloader is built with
CONFIG_ANDROID_AB), but we can delete the unused 'b' slot as the update
of microdroid is not done by writing into the disk, but updating the
virt APEX that has the microdroid image.

Bug: N/A
Test: atest MicrodroidHostTestCases
Change-Id: Ie5871bf1b665e2cc564af54338043ae482ca01bb
4 files changed