Fix lint warning introduced in D49715284 (#526)

Summary:
Pull Request resolved: #526

D49715284 removed some code from this file but didn't remove an import that's no longer needed. This triggered a lint error in OSS CI.

Reviewed By: angelayi

Differential Revision: D49755124

fbshipit-source-id: 94c73ee0446951003932126837fa4e87ba60b2b0

Author: dbort

exir/common.py

@@ -5,7 +5,6 @@
 # LICENSE file in the root directory of this source tree.
 
 # pyre-strict
-import copy
 import logging
 import re
 import sys