[phobos] Removing std.stdio.File.popen()

[Adam Ruppe]

I actually use it (which is why I duplicated your bug), but am OK with
removing it, since it is easy enough to get at anyway. For a while, I
did a separate extern(C) for pclose anyway!

However, I don't think something being POSIX only is a good reason to
remove something. D should take advantages of whatever platform it is
on. Portability is good when you can have it, but it shouldn't be a
function killer alone.

But again, in this specific case, it is easy enough to call the C
directly, so it could go.