remove references to flake8-2 and flake8-3

Summary:
The internal `flake8-2` and `flake8-3` binaries are now deprecated
copies of the main `flake8` binary. This removes remaining references
to the deprecated binaries in preparation for them being removed
in future deployments of flake8.

maintaining_tech_debt

Reviewed By: lisroach, vaibhava0

Differential Revision: D37766758

fbshipit-source-id: ee89cecb0e295b8037843e7e28344b156a847554

Author: amyreese

dev/linter.sh

@@ -26,8 +26,8 @@ echo "Running black ..."
 black -l 100 .
 
 echo "Running flake8 ..."
-if [ -x "$(command -v flake8-3)" ]; then
-  flake8-3 .
+if [ -x "$(command -v flake8)" ]; then
+  flake8 .
 else
   python3 -m flake8 .
 fi