[libc++] Remove nonexistent directory from check-generated-output (#111746)

The libcxx/benchmarks directory was moved to libcxx/test/benchmarks,
which is already checked by that grep command.

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -238,7 +238,7 @@ check-generated-output)
     # Reject patches that introduce non-ASCII characters or hard tabs.
     # Depends on LC_COLLATE set at the top of this script.
     set -x
-    ! grep -rn '[^ -~]' libcxx/include libcxx/src libcxx/test libcxx/benchmarks \
+    ! grep -rn '[^ -~]' libcxx/include libcxx/src libcxx/test \
            --exclude '*.dat' \
            --exclude '*unicode*.cpp' \
            --exclude '*print*.sh.cpp' \