Add functionality behind annotations eraser

[hannahker]

See plotly/plotly.js#6679

Based on an updated understanding of how the erase mode works, the ui will need to be updated. The eraser actually only works when you have a selected shape. We should re-implement this by:

• removing the eraser from the toolbar (as it's not a "mode" per-se)
• adding a callback attached to the "delete" keybind which will delete an active shape.

The syntax of the clientside callback also needs to be updated slightly as the version referenced in the PR doesn't work.

This is the (hacky) workaround:

    """
    function deleteShape(_, graph_id) {
        var gd = document.getElementsByClassName('js-plotly-plot')[0];
        Plotly.deleteActiveShape(gd)
        return dash_clientside.no_update
    }
    """,