All EVP_PKEY types return 1 on that. (It can go away entirely when
EVP_PKEY_HMAC is gone.) This removes a place internally where we're sensitive
to the failure code.
Change-Id: Ic6cda2da9337ba7ef1c66a18e40c5dcc44fcf840
Reviewed-on: https://boringssl-review.googlesource.com/3871
Reviewed-by: Adam Langley <agl@google.com>