tox: remove checkqa-mypy environment

We run mypy through pre-commit, and we don't keep duplicate targets in
tox for all of the other linters. Since this adds some (small)
maintenance overhead, remove it.

.pre-commit-config.yaml

@@ -49,7 +49,7 @@ repos:
     hooks:
     -   id: python-use-type-annotations
 -   repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v0.790  # NOTE: keep this in sync with setup.cfg.
+    rev: v0.790
     hooks:
     -   id: mypy
         files: ^(src/|testing/)