[libc++] Removed unused internal helper functions from the midpoint.h header (#175514)

libcxx/include/__numeric/midpoint.h

diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 8b379f47..d8e73ab 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -52,11 +52,6 @@
   return __a + std::midpoint(ptrdiff_t(0), __b - __a);
 }
 
-template <typename _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr int __sign(_Tp __val) {
-  return (_Tp(0) < __val) - (__val < _Tp(0));
-}
-
 template <typename _Fp>
 _LIBCPP_HIDE_FROM_ABI constexpr _Fp __fp_abs(_Fp __f) {
   return __f >= 0 ? __f : -__f;