Revert "Sum and Mean only support F32"
This was over-zealous, mean does in fact support quantization, sum does not however. This was triggered when enabling support for models with dynamic tensors.

I will revert the mean change internally as it will undoubtedly cause some tests to fail. Should land in a few days.  Thanks for the fast reply, I wasn't aware that sum doesn't yet support quantization. Support for generic reduction (any axis combination supported) for f32 and signed quantized is being worked on and should land in the next few weeks.
This PR reverts commit 990b87a2d40a236517b758e27c850679f0a87213.

990b87a2d40a236517b758e27c850679f0a87213 removed support for integer quantized sum and mean ops. As far as I can tell XNNPACK correctly supports quantized versions of these ops but maybe @alankelly has additional context on why this was removed 990b87a2d40a236517b758e27c850679f0a87213.