Remove (un)mount option for datamedia partitions

When /sdcard is /data/media, it can be formatted but it cannot be
mounted/unmounted independently of /data (aside from creating or
destrying the symlink). This removes /sdcard from the mountable
volumes list, but leaves it as a formattable partition.

Change-Id: If59e4516a12da4589ffb0dbd19702f219260370e
Signed-off-by: garwedgess <garethwilliams21@gmail.com>