fs_mgr: remove configurable zram backing file path

Bug: 142795879
Test: zram_loopback_path never change file location.
Test: Setting zram_backingdev_size on fstab turns on zram writeback.
Change-Id: If244e690405eb925270f767a22e4db95cbd8ba8a
Signed-off-by: Minchan Kim <minchan@google.com>