Do not unconditionally remove MTP storage on unmount

* It is not necessary to Remove_MTP_Storage() for a partition if an
  MTP_Storage_ID has not been assigned.
* The hack to force-set tw_mtp_enabled=1 should no longer be necessary
  now that we're checking whether MTP really needs to be disabled for
  a volume before running Disable_MTP().

Change-Id: I1b7233eedd9da7c6be0c67fc60243f3837105173