bootstrap: the backtrace feature is stable, no need to allow it any more

Author: RalfJung

src/bootstrap/builder.rs

@@ -1556,13 +1556,12 @@ impl<'a> Builder<'a> {
         match mode {
             Mode::ToolBootstrap => {
                 // Restrict the allowed features to those passed by rustbuild, so we don't depend on nightly accidentally.
-                // HACK: because anyhow does feature detection in build.rs, we need to allow the backtrace feature too.
-                rustflags.arg("-Zallow-features=binary-dep-depinfo,backtrace");
+                rustflags.arg("-Zallow-features=binary-dep-depinfo");
             }
             Mode::ToolStd => {
                 // Right now this is just compiletest and a few other tools that build on stable.
                 // Allow them to use `feature(test)`, but nothing else.
-                rustflags.arg("-Zallow-features=binary-dep-depinfo,test,backtrace,proc_macro_internals,proc_macro_diagnostic,proc_macro_span");
+                rustflags.arg("-Zallow-features=binary-dep-depinfo,test,proc_macro_internals,proc_macro_diagnostic,proc_macro_span");
             }
             Mode::Std | Mode::Rustc | Mode::Codegen | Mode::ToolRustc => {}
         }