Remove older migration code.

We used to offer to migrate pre-multi-user devices to the new
multi-user layout around JB, but the new layout has been the default
for many years now, so it's safe to remove this logic.

All core directory creation is now handled by init.rc where FBE
policies can be setup correctly, so we shouldn't be in the business
of doing system bootstrapping here.

Remove some lingering FBE debugging code.

Bug: 26466827
Change-Id: I1dfe9c8fc02b64013a5c45cea99bf8f61f7d1d5b