Remove `build/` suppression

The authoring of NuGet packages quite often include `build` folders, and when this is .gitignore'd it's very often the source of bugs in files that don't get checked in.

I've never seen a Visual Studio project that builds to a `build` folder (`bin` is the default name). As this is a Visual Studio template file, and we have real projects that include `build` folders that include source code, I recommend we remove suppression of this folder.