Revert "ext4: make mb_optimize_scan performance mount option work with extents"

This reverts commit 3c65b730.

The reverted commit is likely to also be reverted upstream, but with a
performance regression like this I don't want to wait for that.

See: https://github.com/raspberrypi/linux/issues/5097



Signed-off-by: Phil Elwell <phil@raspberrypi.com>