acme: simplify TLS-SNI challenge cert

There's no need to PEM-encode key/cert pair to create a tls.Certificate.

All tls.X509KeyPair does is decode back to DER and verify that the
key corresponds to the cert. But we already know it does.

Change-Id: Icccdaf7fd86317147476fb03cbc71ee7b4a06edb
Reviewed-on: https://go-review.googlesource.com/26694
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed