Clutter grabs (Pt. 3)

(this MR depends on !2099 (merged) and !2068 (merged))

Now that clutter_input_device_grab(), clutter_input_device_grab_sequence() and meta_plugin_begin_modal() are unused, these can be simply removed.

This just leaves 2 adversaries:

• Wayland grab interfaces
• WM move/resize operations

that will be tackled in future series.

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Should be merged together with gnome-shell!2046 (merged)