quantization: remove most fp16 configs from fbgemm/qnnpack
Summary:
The fbgemm and qnnpack backends mostly support ops with quint8 activations.
Historically, the default backend config has included ops with fp16 activations
for other backends. This PR keeps the old config under a different name to keep
the functionality tested, and makes the default config match fbgemm/qnnpack ops.
Test plan:
```
python test/test_quantization.py -k TestQuantizeFx
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78528
Approved by: https://github.com/andrewor14
