Remove flags handled by cpp_std am: 6a63a039e7 am: 2fef0dcf40 am: 4fbb6d7cbb

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/grpcio-sys/+/1466843

Change-Id: Ib377871636d87c49fe981f24e39c0b6d8e166305