crypto: lib/poly1305 - 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: 1f81c582 ("crypto: arm/poly1305 - remove redundant shash algorithm")
Fixes: f4b1a73a ("crypto: arm64/poly1305 - remove redundant shash algorithm")
Fixes: 378a337a ("crypto: powerpc/poly1305 - implement library instead of shash")
Fixes: 21969da6 ("crypto: x86/poly1305 - remove redundant shash algorithm")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>