Automatic merge from submit-queue
Remove duplicated ECDHE key handling
This PR removes the duplicated ECDHE private key handling. `x509.CreateCertificateRequest` picks the signature type for ECDHE keys already (see https://golang.org/src/crypto/x509/x509.go `signingParamsForPublicKey`). Only the RSA key signature needed customization.
It also defers to `CreateCertificateRequest` to return errors on unknown private key types.