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

[Lars Tandle Kyllingstad]

I just noticed that std.stdio.File.popen() is POSIX-only.  Taking that
into account, along with the fact that it is buggy (issue 3157), does
anyone mind if I remove it?

It's only used once in Phobos, namely in std.process.shell(), which is
easily implemented by using core.sys.posix.stdio.popen() together with
core.sys.posix.stdio.fdopen() and std.stdio.File.wrapFile().

-Lars