Revert "Removed unnecessary cleaning of Elixir manifests (#423)"

This reverts commit 30bf87d.

Author: josevalim

lib/mix/tasks/gettext.extract.ex

@@ -112,6 +112,9 @@ defmodule Mix.Tasks.Gettext.Extract do
   end
 
   defp force_compile do
+    Mix.Tasks.Compile.Elixir.clean()
+    Enum.each(Mix.Tasks.Compile.Elixir.manifests(), &File.rm/1)
+
     # If "compile" was never called, the reenabling is a no-op and
     # "compile.elixir" is a no-op as well (because it wasn't reenabled after
     # running "compile"). If "compile" was already called, then running