Fix clang-20 toolchain with asio-grpc (#205)

# Changes

I'm removing a preprocessor variable that was introduced to the clang-20
toolchain which was a temporary hack added to the clang-14
implementation specifically for Orion.

# Testing

see swift-nav/orion#9517, previously failing on
libc++ stage, now its passing (currently failing on some formatting,
which can be ignored).

Author: RReichert

cc/toolchains/llvm20/swift_custom_features.bzl

@@ -371,9 +371,6 @@ cxx20_standard_feature = feature(
             flag_groups = ([
                 flag_group(
                     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",
                     ],
                 ),