Remove plugins dialog

Plugins have evolved over time, and for the future, they are hooking points to the application. As they stand now, they aren't toggable features, but rather implementation details of them.

In fact, in the future, the fact that they are plugins is going to be transparent: it's merely a design choice on how to organize feature inside To Do.

Remove the plugins dialog, and all related components. No plugins were harmed while creating this commit.

Closes https://gitlab.gnome.org/GNOME/gnome-todo/-/issues/329