remove use of obsolete golang.org/x/net/context package

This package is an alias for "context", which has been part of
stdlib since go1.7, so should no longer be needed.

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

go.mod

@@ -18,7 +18,6 @@ require (
 	github.com/zmap/zcrypto v0.0.0-20210511125630-18f1e0152cfc
 	github.com/zmap/zlint/v3 v3.1.0
 	golang.org/x/crypto v0.3.0
-	golang.org/x/net v0.2.0
 )
 
 require (
@@ -37,6 +36,7 @@ require (
 	github.com/prometheus/procfs v0.8.0 // indirect
 	github.com/weppos/publicsuffix-go v0.15.1-0.20210511084619-b1f36a2d6c0b // indirect
 	github.com/ziutek/mymysql v1.5.4 // indirect
+	golang.org/x/net v0.2.0 // indirect
 	golang.org/x/sys v0.2.0 // indirect
 	golang.org/x/text v0.4.0 // indirect
 	google.golang.org/protobuf v1.28.1 // indirect

signer/local/local.go

@@ -3,6 +3,7 @@ package local
 
 import (
 	"bytes"
+	"context"
 	"crypto"
 	"crypto/ecdsa"
 	"crypto/elliptic"
@@ -38,7 +39,6 @@ import (
 	zx509 "github.com/zmap/zcrypto/x509"
 	"github.com/zmap/zlint/v3"
 	"github.com/zmap/zlint/v3/lint"
-	"golang.org/x/net/context"
 )
 
 // Signer contains a signer that uses the standard library to