Remove redundant flake8 dependency

Committed via https://github.com/asottile/all-repos
diff --git a/requirements-dev.txt b/requirements-dev.txt
index 2922ef5..cf74d9f 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,7 +1,6 @@
 -e .
 
 coverage
-flake8
 mock
 pre-commit
 pytest