process pipe shouldn't be explicitly closed

[heyitsanthony]

b6b347c seems like a regression; when the child dies it loses the pty and the pipe will close automatically.

With the forced Close, the Interact goroutines seem to be delayed past the cmd.Wait():

Too many goroutines running after all test(s).
3 instances of:
syscall.Syscall(...)
    /usr/lib/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.read(...)
    /usr/lib/go/src/syscall/zsyscall_linux_amd64.go:783 +0x5f
syscall.Read(...)
    /usr/lib/go/src/syscall/syscall_unix.go:161 +0x4d
os.(*File).read(...)
    /usr/lib/go/src/os/file_unix.go:228 +0x53
os.(*File).Read(...)
    /usr/lib/go/src/os/file.go:95 +0x8a
io.copyBuffer(...)
    /usr/lib/go/src/io/io.go:380 +0x247
io.Copy(...)
    /usr/lib/go/src/io/io.go:350 +0x64
created by github.com/coreos/etcd/vendor/github.com/thomasrooney/gexpect.(*ExpectSubprocess).Interact
    /home/anthony/go/src/github.com/coreos/etcd/vendor/github.com/thomasrooney/gexpect/gexpect.go:377 +0x145