test/nil.go: delete on nil maps no longer panics

Fix the build.

R=golang-dev
CC=golang-dev
https://golang.org/cl/6946044

test/nil.go

@@ -151,9 +151,6 @@ func maptest() {
 	shouldPanic(func() {
 		m[2] = 3
 	})
-	shouldPanic(func() {
-		delete(m, 2)
-	})
 }
 
 // nil slice