Revert "ext4fs: Add ext4 extent cache for read operations"

This reverts commit fc0fc50f38a4d7d0554558076a79dfe8b0d78cd5.

The author has asked on the mailing list that we revert this for now as
it breaks write support.

Reported-by: Łukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Tom Rini <trini@ti.com>