[Quant][fx] Remove unused is_qat args in prepare_fx (#96631)
Test Plan:
python test/test_quantization.py TestQuantizeFx
Reviewers: vkuzo, jcaip
Subscribers: vkuzo, jcaip
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96631
Approved by: https://github.com/vkuzo
