vold no longer does MS_MOVE; remove tmpfs.

MS_MOVE was used when staging external storage devices, which no
longer occurs.  In fact, having a writable tmpfs was masking a vold
bug around moving apps to SD cards.

Bug: 11175082
Change-Id: Ib2d7561c3a0b6fde94f651a496cb0c1f12f88d96