helpers/derhelpers: remove support for go1.12

go1.12 has reached EOL when go1.14 was released in 2020, so it
should be safe to remove now.

Also removing uses of golang.org/x/crypto/ed25519, which is now part of stdlib:

    Beginning with Go 1.13, the functionality of this package was moved to the
    standard library as crypto/ed25519. This package only acts as a compatibility
    wrapper.

This partially reverts e45ead2 / 81e5473

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

helpers/derhelpers/derhelpers-legacy.go

@@ -1,5 +1,3 @@
-// +build go1.13
-
 // Package derhelpers implements common functionality
 // on DER encoded data
 package derhelpers

helpers/derhelpers/derhelpers.go

@@ -2,11 +2,10 @@ package derhelpers
 
 import (
 	"crypto"
+	"crypto/ed25519"
 	"crypto/x509/pkix"
 	"encoding/asn1"
 	"errors"
-
-	"golang.org/x/crypto/ed25519"
 )
 
 var errEd25519WrongID = errors.New("incorrect object identifier")

helpers/derhelpers/ed25519.go

@@ -2,10 +2,9 @@ package derhelpers
 
 import (
 	"bytes"
+	"crypto/ed25519"
 	"encoding/pem"
 	"testing"
-
-	"golang.org/x/crypto/ed25519"
 )
 
 var testPubKey = `-----BEGIN PUBLIC KEY-----