ext4: get rid of unused DEFAULT_MB_OPTIMIZE_SCAN

After recent changes to the mb_optimize_scan mount option
the DEFAULT_MB_OPTIMIZE_SCAN is no longer needed so get
rid of it.

Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20220315114454.104182-1-ojaswin@linux.ibm.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>