ci: remove dead RETRY_CLEAN_CMD from bench.sh

retry-build.sh no longer has a RETRY_CLEAN_CMD mechanism; the variable
was a no-op that misled readers into thinking it controlled retry cleanup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/phoenix/bench.sh

@@ -22,7 +22,7 @@ fi
 rm -rf build
 
 source .github/scripts/retry-build.sh
-RETRY_CLEAN_CMD="./mfc.sh clean" retry_build ./mfc.sh build -j $n_jobs $build_opts || exit 1
+retry_build ./mfc.sh build -j $n_jobs $build_opts || exit 1
 
 ./mfc.sh bench $bench_opts -j $n_jobs -o "$job_slug.yaml" -- -c phoenix-bench $device_opts -n $n_ranks