[PATCH] [MTD] block2mtd: remove readahead code

Over the years there was a slow trickle of complaints against the readahead
code.  Most of them concerned performance, Peter Zijlstra stumbled over it
when working unrelated changes and I believe there was an actual bug report.
Oh, Andrew Morton also complained about duplicating code from mm/readahead.c.

It is just not worth it.  On flash media like usb sticks, readahead will
make things go slow - very slow.  On spinning disks, readahead may be a
win, but this is definitely not the place to add it.

Signed-off-by: Jörn Engel <joern@lazybastard.org>