Don't provide rust-demangler to run-make tests

The demangler was only needed by coverage tests, but those tests were migrated
into their own custom test mode in rust-lang#112300.

This avoids having to build the demangler just for run-make tests. It will
still be built as needed by run-coverage tests or for other purposes.

src/bootstrap/src/core/build_steps/test.rs

@@ -1567,7 +1567,7 @@ note: if you're sure you want to do this, please open an issue as to why. In the
             cmd.arg("--coverage-dump-path").arg(coverage_dump);
         }
 
-        if mode == "run-make" || mode == "run-coverage" {
+        if mode == "run-coverage" {
             let rust_demangler = builder
                 .ensure(tool::RustDemangler {
                     compiler,

src/tools/compiletest/src/runtest.rs

@@ -3546,10 +3546,6 @@ impl<'test> TestCx<'test> {
             cmd.env("RUSTDOC", cwd.join(rustdoc));
         }
 
-        if let Some(ref rust_demangler) = self.config.rust_demangler_path {
-            cmd.env("RUST_DEMANGLER", cwd.join(rust_demangler));
-        }
-
         if let Some(ref node) = self.config.nodejs {
             cmd.env("NODE", node);
         }