Remove unused method namespaced_file_path

[mtsmfm]

$ git grep namespaced_file_path
railties/lib/rails/generators/named_base.rb:        def namespaced_file_path
railties/lib/rails/generators/named_base.rb:          @namespaced_file_path ||= namespaced_class_path.join("/")

[rails-bot]

r? @kaspth

(@rails-bot has picked a reviewer for you, use r? to override)

[eileencodes]

For reference, the code using this method was removed in 595fa6e