Remove the chdir call that was already moved to the ancestor

528b4b367ed2a4026c321e2f2e503ace4548b9d2 moved the chdir definition to
the parent class and call it in the parent's stage. chdir was getting
called twice for archives with two nested directories.

Library/Homebrew/download_strategy.rb

@@ -197,11 +197,6 @@ class AbstractFileDownloadStrategy < AbstractDownloadStrategy
     @temporary_path = Pathname.new("#{cached_location}.incomplete")
   end
 
-  def stage
-    super
-    chdir
-  end
-
   private
 
   def ext