ANDROID: Squashfs: remove the FILE_CACHE option

FILE_DIRECT is working fine and offers faster results and lower memory
footprint.

Removing FILE_CACHE makes our life easier because we don't have to
maintain 2 differents function that does the same thing.

Change-Id: Ib11dfeb6847fb79d166e10686a95eabb8c65710a
Signed-off-by: Adrien Schildknecht <adriens@google.com>