Remove BIO_dummy, it's old cruft

Reviewed-by: NRich Salz <rsalz@openssl.org>

include/openssl/crypto.h

@@ -188,9 +188,6 @@ int CRYPTO_atomic_add(int *val, int amount, int *ret, CRYPTO_RWLOCK *lock);
 # define CRYPTO_MEM_CHECK_ENABLE  0x2   /* Control and mode bit */
 # define CRYPTO_MEM_CHECK_DISABLE 0x3   /* Control only */
 
-/* predec of the BIO type */
-typedef struct bio_st BIO_dummy;
-
 struct crypto_ex_data_st {
     STACK_OF(void) *sk;
 };