Allow libc++'s auto std detection.

We've removed enough of our custom logic that we can rely on the
automatic detection now, rather than having to remember to update this
(which we haven't been remembering).

Test: ndk/run_tests.py --filter libc++ --rebuild
Bug: None
Change-Id: Ia8090379b56f798c745710abcd1f990aff541d25
diff --git a/libcxx/test/lit.site.cfg b/libcxx/test/lit.site.cfg
index 9c49cd4..88b1a79 100644
--- a/libcxx/test/lit.site.cfg
+++ b/libcxx/test/lit.site.cfg
@@ -20,7 +20,6 @@
 
 config.cxx_under_test = os.path.join(
     ndk_path, "toolchains/llvm/prebuilt", host_tag, "bin/clang++")
-config.std = "c++14"
 config.libcxx_src_root = libcxx_src
 config.libcxx_obj_root = libcxx_install
 config.test_source_root = os.path.join(libcxx_src, 'test')
diff --git a/libcxx/utils/libcxx/ndk/test/config.py b/libcxx/utils/libcxx/ndk/test/config.py
index dcfe4b4..bd12ede 100644
--- a/libcxx/utils/libcxx/ndk/test/config.py
+++ b/libcxx/utils/libcxx/ndk/test/config.py
@@ -51,9 +51,13 @@
         self.cxx.link_flags.append('-Wl,--exclude-libs,libgcc.a')
 
     def configure_features(self):
-        self.config.available_features.add(self.get_lit_conf('std'))
         self.config.available_features.add('long_tests')
         self.config.available_features.add('c++filesystem-disabled')
+        self.config.available_features.add('libcpp-no-concepts')
+
+    def configure_modules(self):
+        # Not something we support yet.
+        pass
 
     def get_test_format(self):
         # Note that we require that the caller has cleaned this directory,