Revert "fs_mgr: Add support for A/B partitions"

The original CL landed on AOSP in 8bb2f36a in August.
It was then removed and superseded by 227ef3c5 in September.
However, 8bb2f36a was then cherry-picked onto mnc-dr2-dev and
automerged back into mnc-dr2-dev-plus-aosp.

This revert CL fixes an issue that was causing init to hang
before mounting any filesystems on edison (and probably
other platforms).

This reverts commit ebff8b97, reversing
changes made to d254d33e.

Change-Id: Ia5b35217c1f4730bccb7374c24089e439d21752d