dependency: return test deps on all levels

Filtering for this is now handled properly in places where this function is used.

In some scenarios, we _want_ test deps on more than the first level (Homebrew.args.include_formula_test_deps?).

Library/Homebrew/dependency.rb

@@ -89,9 +89,6 @@ class Dependency
       deps.each do |dep|
         next if dependent.name == dep.name
 
-        # we only care about one level of test dependencies.
-        next if dep.test? && @expand_stack.length > 1
-
         case action(dependent, dep, &block)
         when :prune
           next