context, registry/handlers: remove net/context references

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/handlers/context.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 
@@ -10,7 +11,6 @@ import (
 	"github.com/docker/distribution/registry/api/v2"
 	"github.com/docker/distribution/registry/auth"
 	"github.com/opencontainers/go-digest"
-	"golang.org/x/net/context"
 )
 
 // Context should contain the request specific context for use in across