crypto: adiantum - Drop support for asynchronous xchacha ciphers

crypto: adiantum - Drop support for asynchronous xchacha ciphers

This feature isn't useful in practice.  Simplify and streamline the code
in the synchronous case, i.e. the case that actually matters, instead.

For example, by no longer having to support resuming the calculation
after an asynchronous return of the xchacha cipher, we can just keep
more of the state on the stack instead of in the request context.

Link: https://lore.kernel.org/r/20251211011846.8179-10-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>