[quant][be] Remove special casing for getitem in prepare (#90393)
Summary:
This PR cleans up previous special casing for getitem, it should be configured through BackendConfig
Test Plan:
python test/test_quantization.py TestQuantizeFx
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Subscribers:
Tasks:
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Differential Revision: [D41846185](https://our.internmc.facebook.com/intern/diff/D41846185)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90393
Approved by: https://github.com/andrewor14
