OCL: [photo_denoising]Remove unnecessary local variable "size".

It should also be better for optimizing when compiling kernel.
It could improve the following cases:
OCL_Photo_DenoisingGrayscale.DenoisingGrayscale
OCL_Photo_DenoisingColored.DenoisingColored

Signed-off-by: Yan Wang <yan.wang@linux.intel.com>