crypto/x509: fix duplicate import

Updates dave/dst#45.

Change-Id: I165e6b3d002407a33908bf90a66ad01f8003b260
Reviewed-on: https://go-review.googlesource.com/c/go/+/250497
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
diff --git a/src/crypto/x509/x509.go b/src/crypto/x509/x509.go
index 8ce57fb..49ac059 100644
--- a/src/crypto/x509/x509.go
+++ b/src/crypto/x509/x509.go
@@ -14,7 +14,6 @@
 	"crypto/elliptic"
 	"crypto/rsa"
 	"crypto/sha1"
-	_ "crypto/sha1"
 	_ "crypto/sha256"
 	_ "crypto/sha512"
 	"crypto/x509/pkix"