strip: remove -b/--backup codepaths cset ba3bc6474bbf has removed this option. This commit just tidies the code that was associated to it. It also fixes the internal calls to the strip() function. Before this change, any function that thought it would want as a final safety to keep a partial backup bundle (bundling changes not linearly related to the current change being stripped), had to explicitly pass a backup="strip" option. With this change, these backups are always kept in case of an exception and always removed if there is no exception. Only full backups can be specified with backup=True or no full backups with backup=False.