fastbootd: Delete all VAB partitions during "flashall".

With VAB we encourage a smaller super partition, so make sure we delete
"other" slot partitions during flashing. Otherwise, we may not have
enough space in super.

Bug: 195930130
Test: set_active, flashall, ensure other slot is deleted
Change-Id: Ic6a4f60e8f4c7abb8af7b38228753ad8ed85eedf