Remove unused headers and functions (#3316)

Remove unused headers and functions (#3316)

This patch will remove all unused headers and functions from following files:
CLActivationLayerExKernel.cpp
CLBatchToSpaceNDKernel.cpp
CLDepthToSpaceKernel.cpp
CLEqualKernel.cpp
CLGatherKernel.cpp
CLNegKernel.cpp
CLPixelWiseDivisionKernel.cpp
CLReduceMaxKernel.cpp
CLReduceSumKernel.cpp
CLReductionMeanKernel.cpp
CLSpaceToDepthKernel.cpp
CLSquaredDifferenceKernel.cpp
CLStridedSliceKernel.cpp

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