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.