Clobber/delete destination ftp files as needed

The comment doesn't match code any more.  You might also do:
```
destination_file = "{}/{}".format(ftp_dirname, file_base)
```
and then use that in the two places below.  Most of the time if I use a slightly non-trivial expression at least twice I like to factor it into a new variable so it's obvious they are the same.