EVP_CIPHER_CTX_cleanup cannot fail.

There is exactly one implementation and it doesn't fail. Plus a cleanup
function that can fail is very bad manners; the caller has no choice but to
leak at that point.

Change-Id: I5b524617ef37bc7d92273472fa742416ea7dfd43
Reviewed-on: https://boringssl-review.googlesource.com/3564
Reviewed-by: Adam Langley <agl@google.com>