Remove unused GPTQ code (#2844)

Summary:
Pull Request resolved: #2844

Removing GPTQ and Handlers in quantize.py in favor of AO

Reviewed By: jerryzh168

Differential Revision: D55554093

fbshipit-source-id: d773f23956c44719c83cdb79d301370ea7bc9542

Author: Jack-Khuu