Revert "Use 64 KiB readahead"

This reverts commit 2335adc3c3.

In benchmarks, disabling readahead improves both reads, writes,
random reads, and random writes by up to 4% in IOPS during an
IOZone test with Android stopped and caches cleared before each
run.

Signed-off-by: Tyler Nijmeh <tylernij@gmail.com>

system/bin/ktweak

@@ -172,7 +172,7 @@ do
 		fi
 	done
 	write "${queue}iostats" 0
-	write "${queue}read_ahead_kb" 64
+	write "${queue}read_ahead_kb" 0
 	write "${queue}nr_requests" 64
 done