clean up default-stanzas from extended_states on write

clean up default-stanzas from extended_states on write

The existing cleanup was happening only for packages which had a status
change (install -> uninstalled) which is the most frequent but no the
only case – you can e.g. set autobits explicitly with apt-mark.

This would leave stanzas in the states file declaring a package to be
manually installed – which is the default value for a package not listed
at all, so we can just as well drop it from the file.