[libc++] Clean up the no-unicode CI job

It was added after we changed the way the CI jobs are run, in particular
how they are pinned down to Linux instances only. As a result, the job
would sometimes run on Mac machines, which we're trying to keep only for
jobs that absolutely need it due to capacity concerns.

libcxx/utils/ci/buildkite-pipeline.yml

@@ -388,10 +388,12 @@ steps:
       - "**/test-results.xml"
     agents:
       queue: "libcxx-builders"
+      os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
+    timeout_in_minutes: 120
 
   - label: "Benchmarks"
     command: "libcxx/utils/ci/run-buildbot benchmarks"

libcxx/utils/ci/run-buildbot

@@ -340,8 +340,6 @@ generic-no-localization)
     check-cxx-cxxabi
 ;;
 generic-no-unicode)
-    export CC=clang
-    export CXX=clang++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-no-unicode.cmake"
     check-cxx-cxxabi