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