remove OS.File references

remove OS.File references

osfile.jsm was removed by bugzil.la/1772262. This method is not used any
more, so we can just remove it. the modern alternative would be the
following, but that only works on mozilla>=85.
    return IOUtils.writeUTF8(file, this.serializeToString());

osfile.jsm was loaded lazily (and in our case therefore never), so we
didn't die immediately upon import, fortunately.

see also: https://hg.mozilla.org/mozilla-central/diff/a11c616997d802e44d1afc0215bfbffac865179e/toolkit/components/passwordmgr/test/LoginTestUtils.jsm