Drop the `fails_paying_for_bolt12_invoice` test

Drop the `fails_paying_for_bolt12_invoice` test

`fails_paying_for_bolt12_invoice` tests that we fail to send a
payment if the router returns `Ok` but includes a bogus route (one
with 0-length paths). While this marginally increases our test
coverage, in the next commit we'll be testing that all routes
round-trip serialization, which fails here as bogus routes are not
supported in deserialization.

Because this isn't particularly critical test coverage, we simply
opt to drop the test entirely here.