Fix clang-20 toolchain with asio-grpc

Author: RReichert

cc/toolchains/llvm20/swift_custom_features.bzl

@@ -373,7 +373,6 @@ cxx20_standard_feature = feature(
                     flags = [
                         # This is a workaround for memory_resource being experimental only
                         # in the llvm-14 libc++ impelmentation.
-                        "-DSWIFTNAV_EXPERIMENTAL_MEMORY_RESOURCE",
                         "-D_LIBCPP_ENABLE_CXX20_REMOVED_FEATURES",
                     ],
                 ),