importlib-metadata has type annotations

In #12326 I added this config to get the tox mypy job to pass. It failed
on a630a0a (see
https://github.com/matrix-org/synapse/runs/5740930065?check_suite_focus=true)
because tox hadn't installed importlib_metadata at all.

Now that we're using the locked poetry environment, we don't have this
problem any more.

mypy.ini

@@ -250,9 +250,6 @@ ignore_missing_imports = True
 [mypy-ijson.*]
 ignore_missing_imports = True
 
-[mypy-importlib_metadata.*]
-ignore_missing_imports = True
-
 [mypy-jaeger_client.*]
 ignore_missing_imports = True