Undo metadata workaround in flight-client

[bclozel]

We've introduced a temporary workaround in eb8ad0af9a547eb33518b6c7bbd948d058007a38 to manually encode the routing metadata with the expected format.

Once the rsocket-js library has been updated to support composite metadata, we should be able to undo that workaround and possibly add other metadata, such as security-related information.