rubocops/lines: Clean up an old TODO

- I considered writing a cop for this, but it's not worth it:
  there are no `[:test, :build]` occurrences in Core and this
  Rust rule only applies in Core formulae.

Library/Homebrew/rubocops/lines.rb

@@ -938,9 +938,7 @@ def audit_formula(formula_nodes)
             end
           end
 
-          # TODO: Enforce order of dependency types so we don't need to check for
-          #       depends_on "rustup" => [:test, :build]
-          [:build, [:build, :test], [:test, :build]].each do |type|
+          [:build, [:build, :test]].each do |type|
             find_method_with_args(body_node, :depends_on, "rustup" => type) do
               problem "Formulae in homebrew/core should use 'depends_on \"rust\" => #{type}' " \
                       "instead of '#{@offensive_node.source}'." do |corrector|