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.

Signed-off-by: Adrien Schildknecht <adriens@google.com>
Change-Id: I2ac4701a2d4aa054a2753af2f2f1011a935f1d87