test: move map delete test to nil.go

No need for a separate test for this.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/6941045

test/fixedbugs/issue4535.go

-// run
-
-// Copyright 2012 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package main
-
-func main() {
-	var m map[int]int
-	delete(m, 0)
-}

test/nil.go

@@ -147,6 +147,9 @@ func maptest() {
 		panic(v)
 	}
 
+	// can delete (non-existent) entries
+	delete(m, 2)
+
 	// but cannot be written to
 	shouldPanic(func() {
 		m[2] = 3