Remove Meltano section header/footer when uninstalling every entry

[WillDaSilva]

The code that currently handles uninstall --all is:

        if uninstall_all:
            # XXX: This leaves behind the "begin/end Meltano section" comments
            predicate = lambda _: True

There is no special case in the setter for entries (or the setter for crontab) that removes the header and footer when no entries remain.