COMPMID-3852: Fix NEReduction window

COMPMID-3852: Fix NEReduction window

ReductionOperations splits the kernel for scheduling on the X
dimension when reduction axis is > 0.
By setting the execution window to be unit one in the X dimension the
execution was always restricted to a single thread.
Alters the window to enable multi-threading

Signed-off-by: Georgios Pinitas <georgios.pinitas@arm.com>
Change-Id: Idcbe2b78957678310bb8e895969f01de972d3667
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4389
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>