Fix typing in callback_on_mock_put

[dperl-dls]

callback_on_mock_put had an incorrect type annotation which was causing some errors, removing it lets the type checker correctly note it as:

(function) def callback_on_mock_put(
    signal: Signal[Unknown],
    callback: (T@callback_on_mock_put) -> None
) -> _GeneratorContextManager[None]

[evalott100]

lgtm