remove null-stream (#670)

It's a follow up to https://github.com/nushell/nushell/pull/11070

stdlib-candidate/null-stream.nu

@@ -1,10 +0,0 @@
-# Examples
-#     redirect to the null stream
-#     > echo foo out> (null-stream)
-def null-stream []: nothing -> string {
-    if $nu.os-info.name  == "windows" {
-        "NUL:"
-    } else {
-        "/dev/null"
-    }
-}