USB: serial: remove changelogs and old todo entries

Remove remaining changelogs from file headers (can still be retrieved
through git).
Remove even older changelog entries stored in Changelog.history.
Remove outdated todo entries from belkin_sa.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>