This is the same deprecation, but one level up. For the moment, we
still support invoices with a `h` field (where description will be
necessary) but that will be removed once this option is removed.
Note that I just changed pylightning without backwards compatibility,
since the field was unlikely to be used, but we could do something
more complex here?
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>