fastboot: Delete logical "other" partitions on retrofit devices.

On retrofit devices, if both slots contain dynamic partition builds,
then "flashall" will attempt to write secondary images to dynamic
partitions in the other slot. At worst, this can fail with an error. At
best, it will result in the "other" partition not being mounted on first
boot.

This patch therefore deletes logical partitions for secondary images, on
retrofit devices only. On a Pixel device on the "b" slot, this means
"system_a" and "vendor_a" will be deleted before flashing, and therefore
system_other and vendor_other will be flashed to physical partitions
instead.

Bug: 120034852
Test: fastboot set_active a
      fastboot flashall
      fastboot set_active b
      fastboot flashall

Change-Id: I6affe9a6c639b0495bffc77fcf20f329b86ad159