remove obsolete assignment

Author: tak1n

kernel/common/process_mirror.rb

@@ -185,7 +185,7 @@ def convert_to_fd(obj, target)
               [obj[0], File::RDONLY, 0644]
             when 2
               if obj[0] == :child
-                fd = convert_to_fd obj[1], target
+                convert_to_fd obj[1], target
               else
                 [obj[0], convert_file_mode(obj[1]), 0644]
               end