Remove deprecations

[fonsp]

We are going to release 0.20.0 soon. That means we can remove some deprecations

• init_with_file_viewer
• downstream_cells_map
• upstream_cells_map(cell::Cell, notebook::Notebook)
• delete_vars
• where_referenced(notebook::Notebook, topology::NotebookTopology, something)
• where_assigned(notebook::Notebook, topology::NotebookTopology, something)

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="remove-deprecations")
julia> using Pluto