Remove build.rs note about non-public rustc-cfg strings

The build.rs is no longer even published to crates.io, so of course the
cfgs aren't public.

build.rs

@@ -5,9 +5,6 @@ use std::ffi::OsString;
 use std::iter;
 use std::process::{self, Command, Stdio};
 
-// The rustc-cfg strings below are *not* public API. Please let us know by
-// opening a GitHub issue if your build environment requires some way to enable
-// these cfgs other than by executing our build script.
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");