Revert "library/test: always enable unstable features for miri"

This reverts commit 30d7ed4.
It should not be needed any more.

Author: RalfJung

library/test/build.rs

@@ -2,16 +2,11 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-check-cfg=cfg(enable_unstable_features)");
 
-    // Miri testing uses unstable features, so always enable that for its sysroot.
-    // Otherwise, only enable unstable if rustc looks like a nightly or dev build.
-    let enable_unstable_features = std::env::var("MIRI_CALLED_FROM_SETUP").is_ok() || {
-        let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".into());
-        let version = std::process::Command::new(rustc).arg("-vV").output().unwrap();
-        let stdout = String::from_utf8(version.stdout).unwrap();
-        stdout.contains("nightly") || stdout.contains("dev")
-    };
+    let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".into());
+    let version = std::process::Command::new(rustc).arg("-vV").output().unwrap();
+    let stdout = String::from_utf8(version.stdout).unwrap();
 
-    if enable_unstable_features {
+    if stdout.contains("nightly") || stdout.contains("dev") {
         println!("cargo:rustc-cfg=enable_unstable_features");
     }
 }