internal: cleanup remaining x/net/context (#2470)

Documentation/server-reflection-tutorial.md

@@ -16,7 +16,6 @@ For example, to enable server reflection in `example/helloworld`, we need to mak
 --- a/examples/helloworld/greeter_server/main.go
 +++ b/examples/helloworld/greeter_server/main.go
 @@ -40,6 +40,7 @@ import (
-        "golang.org/x/net/context"
         "google.golang.org/grpc"
         pb "google.golang.org/grpc/examples/helloworld/helloworld"
 +       "google.golang.org/grpc/reflection"

test/healthcheck_test.go

@@ -19,6 +19,7 @@
 package test
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"net"
@@ -26,7 +27,6 @@ import (
 	"testing"
 	"time"
 
-	"golang.org/x/net/context"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/connectivity"