Revert part of 67aae9d2.

* rsa-sec-decrypt.c (rsa_sec_decrypt): Merge with
_rsa_sec_decrypt, including input range check.
(_rsa_sec_decrypt): Deleted.
* rsa-internal.h (_rsa_sec_decrypt): Delete declaration.
* testsuite/rsa-sec-decrypt-test.c (rsa_decrypt_for_test): Always
call rsa_sec_decrypt, but don't annotate the ciphertext input as
undefined/secret.