Don't keep file_handle around on windows for mmap

It looks as if the file_handle we keep around in our windows tor_mmap_t is needless. A pseudonymous person who has in the past asked not to be named, provides a patch (attached) to correct this.

I'm making a branch against 0.2.2.x. If this turns out to actually help with Kaspersky or something, let's backport it. Otherwise, I'd propose it for merge into 0.2.3.x.