Remove potentially superfluous attributes in _File class

[linozen]

Since we should have a dataclass containing all the copyright and licensing information when #654 is merged, one might be able to remove the following attributes of the _File class:

class _File:
    """Represent an SPDX file. Sufficiently enough for our purposes, in any
    case.
    """
    def __init__(self, name, spdx_id=None, chk_sum=None):
        ...
        self.license_concluded: str = None
        self.copyright: str = None
        ...

It is likely that this is a larger refactoring.