Also remove some transition step for a recent format change. Together, this
removes the curve hacks in the converter, which can now be purely syntactic.
The RSA ones are still a bit all over the place in terms of sharded vs
combined, so leaving that alone for now.
Change-Id: I721d6b0de388a53a39543725e366dc5b52e83561
Reviewed-on: https://boringssl-review.googlesource.com/30845
Reviewed-by: Adam Langley <agl@google.com>
