message: Delete RTC_WARN_UNUSED_RESULT as no longer used
  All usage was replaced with ABSL_MUST_USE_RESULT
  Cr-Commit-Position: refs/heads/master@{#33020}
diff: #ifndef RTC_BASE_SYSTEM_UNUSED_H_
  #define RTC_BASE_SYSTEM_UNUSED_H_
  -// Annotate a function indicating the caller must examine the return value.
  -// Use like:
  -// int foo() RTC_WARN_UNUSED_RESULT;
  -// To explicitly ignore a result, cast to void.
  -// TODO(kwiberg): Remove when we can use [[nodiscard]] from C++17.
  -#if defined(__clang__)
  -#define RTC_WARN_UNUSED_RESULT __attribute__((__warn_unused_result__))
  -#elif defined(__GNUC__)
  -// gcc has a __warn_unused_result__ attribute, but you can't quiet it by
  -// casting to void, so we don't use it.
  -#define RTC_WARN_UNUSED_RESULT
  -#else
  -#define RTC_WARN_UNUSED_RESULT
  -#endif
  -
  // Prevent the compiler from warning about an unused variable. For example:
  // int result = DoSomething();
  // assert(result == 17);
  