crypto: lib/chacha - restore ability to remove modules

Though the module_exit functions are now no-ops, they should still be
defined, since otherwise the modules become unremovable.

Fixes: 08820553 ("crypto: arm/chacha - remove the redundant skcipher algorithms")
Fixes: 8c28abed ("crypto: arm64/chacha - remove the skcipher algorithms")
Fixes: f7915484 ("crypto: powerpc/chacha - remove the skcipher algorithms")
Fixes: ceba0eda ("crypto: riscv/chacha - implement library instead of skcipher")
Fixes: 632ab097 ("crypto: x86/chacha - remove the skcipher algorithms")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>