`SWEEnv` refactoring tasks

[Kilian Lieret]

• sha from swe-agent is never used
• Remove aliases to self.args (e.g., self.image_name=args.image_name) if they aren't modified later
• clean_multi_line_functions not used?
• self.idx not used?
• self.communicate_output never read?
• self.base_commit is only an alias?
• Make self.reset simply return container, container_obj, record, so that they're never None and mypy shuts up
• Clean up all the confusion between all the different reset, _reset, _init, close, etc.