controllers: disable explicit purge of invalid file

app/controllers/champs/piece_justificative_controller.rb

@@ -9,12 +9,6 @@ class Champs::PieceJustificativeController < ApplicationController
       render :show
     else
       errors = @champ.errors.full_messages
-
-      # Before Rails 6, the attachment was persisted to database
-      # by 'attach', even before calling save.
-      # So until we're on Rails 6, we need to purge the file explicitely.
-      @champ.piece_justificative_file.purge_later
-
       render :json => { errors: errors }, :status => 422
     end
   end