Rendezvous handler code can be simplified after `text/plain` requirement

[anoadragon453]

Context: https://github.com/element-hq/synapse/pull/17122#discussion_r1579294847

The code can likely be simplified a bit now that only Content-Types of text/plain are allowed.

We may also want to enforce this requirement in the type checker, rather than a runtime check, to reduce the chance of this check accidentally being removed in the future, and other code assuming it is there.