update how-to-build.md and delete obsolete tutorials in docs

[kernelbin]

There's already a file called BUILD under the root dir of protobuf, thus we can't mkdir build under Windows, as Windows is usually case-insensitive.

I renamed it protobuf_build.

build-for-VisualStudio.en.md and build-for-VisualStudio.zh.md seems to be obsoleted so I deleted them