Remove duplicate flake8 call in tox, it's covered by pre-commit (#1448)

Author: ulgens

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3{6,7,8,9,10}, flake8, mypy, pre-commit
+envlist = py3{6,7,8,9,10}, mypy, pre-commit
 skipsdist = true
 
 [testenv]
@@ -26,12 +26,4 @@ deps =
 commands =
     mypy graphene
 
-[testenv:flake8]
-basepython = python3.9
-deps =
-    flake8>=4,<5
-commands =
-    pip install --pre -e .
-    flake8 graphene
-
 [pytest]