make.bat: don't show error message if old generated files do not exist

R=golang-dev, alex.brainman, mattn.jp
CC=golang-dev
https://golang.org/cl/5843045

src/make.bat

@@ -29,7 +29,7 @@ if "x%GOROOT_FINAL%"=="x" set GOROOT_FINAL=%GOROOT%
 set DEFGOROOT=-DGOROOT_FINAL="\"%GOROOT_FINAL:\=\\%\""
 
 :: Clean old generated file that will cause problems in the build.
-del /F ".\pkg\runtime\runtime_defs.go"
+del /F ".\pkg\runtime\runtime_defs.go" 2>NUL
 
 echo # Building C bootstrap tool.
 echo cmd/dist