Remove unnecessary headers from arm_compute/core/CL/* (#3371)

Remove unnecessary headers from arm_compute/core/CL/* (#3371)

This patch will remove unnecessary headers included in the following files:

arm_compute/core/CL/kernels/CLActivationLayerExKernel.h
arm_compute/core/CL/kernels/CLArithmeticSubtractionExKernel.h
arm_compute/core/CL/kernels/CLEqualKernel.h
arm_compute/core/CL/kernels/CLGatherKernel.h
arm_compute/core/CL/kernels/CLNegKernel.h
arm_compute/core/CL/kernels/CLPixelWiseDivisionKernel.h
arm_compute/core/CL/kernels/CLReduceMaxKernel.h
arm_compute/core/CL/kernels/CLReductionMeanKernel.h
arm_compute/core/CL/kernels/CLSquaredDifferenceKernel.h
arm_compute/core/CL/kernels/CLStridedSliceKernel.h

Signed-off-by: shubham <shub98.gupta@samsung.com>