Revert "Fix x test lint-docs when download-rustc is enabled"

This was not the correct fix. The problem was two-fold:
- `download-rustc` didn't respect `llvm.assertions`
- `rust-dev` was missing a bump to `download-ci-llvm-stamp`

The first is fixed in this PR and the latter was fixed a while ago. Revert this change to avoid breaking `rpath = false`.

src/tools/lint-docs/src/groups.rs

@@ -39,7 +39,6 @@ impl<'a> LintExtractor<'a> {
     fn collect_groups(&self) -> Result<LintGroups, Box<dyn Error>> {
         let mut result = BTreeMap::new();
         let mut cmd = Command::new(self.rustc_path);
-        cmd.env_remove("LD_LIBRARY_PATH");
         cmd.arg("-Whelp");
         let output = cmd.output().map_err(|e| format!("failed to run command {:?}\n{}", cmd, e))?;
         if !output.status.success() {

src/tools/lint-docs/src/lib.rs

@@ -403,12 +403,6 @@ impl<'a> LintExtractor<'a> {
         fs::write(&tempfile, source)
             .map_err(|e| format!("failed to write {}: {}", tempfile.display(), e))?;
         let mut cmd = Command::new(self.rustc_path);
-        // NOTE: bootstrap sets `LD_LIBRARY_PATH` for building lint-docs itself.
-        // Unfortunately, lint-docs is a bootstrap tool while rustc is built from source,
-        // and sometimes the paths conflict. In particular, when using `download-rustc`,
-        // the LLVM versions can differ between `ci-llvm` and `ci-rustc-sysroot`.
-        // Unset LD_LIBRARY_PATH here so it doesn't interfere with running the compiler.
-        cmd.env_remove("LD_LIBRARY_PATH");
         if options.contains(&"edition2015") {
             cmd.arg("--edition=2015");
         } else {