fx quant: remove some dead code in observer insertion (redo) (#59799)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59799
This is a redo of #58574, easier to create a new PR than to fix rebase
conflicts, as there have been a large number of refactors to the
underlying code.
Removes some code which was incorrectly added by #57519 but never
actually used for anything.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D29031955
fbshipit-source-id: f407d181070cb283382965952821e3647c705544
