Revert "mm: backing-dev: Take a reference to the bdi in use to prevent UAF"

This broke installing two zips without rebooting in between.

[   34.799566] mount: File exists
[   34.799588] fuse_sideload umount failed: Invalid argument
[   34.799665] minadbd E 04-06 01:15:10  1223  1230 minadbd_services.cpp:120] Failed to start fuse
[   37.801699] minadbd E 04-06 01:15:13  1223  1230 minadbd_services.cpp:164] poll() failed: Invalid argument
[   48.800155]
[   48.800170] Timed out waiting for fuse to be ready.
[   48.800176]
[   48.900194] I:Command 0 finished with 0
[   48.900229] ERROR:   recovery: Failed to write message STAT: Broken pipe

This reverts commit d6f18f6a.

Change-Id: Ie0b1ca0dd9c533d69e93a485a885a9867aa1b267