Remove flake8 F821 checks for stubs (#14367)

F821 is only checked as a test for the F821 monkeypatch functionality in
flake8-pyi, but that will be removed.

Author: srittau

.flake8

@@ -1,7 +1,6 @@
 [flake8]
 # Y: Flake8 is only used to run flake8-pyi, everything else is in Ruff
-# F821: Typeshed is a testing ground for flake8-pyi, which monkeypatches F821
-select = Y, F821
+select = Y
 # Ignore rules normally excluded by default
 extend-ignore = Y090,Y091
 per-file-ignores =