Remove unused EncodePlacementArenaOffsets function.

PiperOrigin-RevId: 877976526

Author: ClaytonKnittel

src/google/protobuf/generated_message_util.h

@@ -390,19 +390,6 @@ inline void AddToRepeatedPtrField(InternalVisibility visibility,
   dest.InternalAddWithArena(visibility, arena, std::move(value));
 }
 
-constexpr absl::optional<uintptr_t> EncodePlacementArenaOffsets(
-    std::initializer_list<size_t> offsets) {
-  uintptr_t arena_bits = 0;
-  for (size_t offset : offsets) {
-    offset /= sizeof(Arena*);
-    if (offset >= sizeof(arena_bits) * 8) {
-      return absl::nullopt;
-    }
-    arena_bits |= uintptr_t{1} << offset;
-  }
-  return arena_bits;
-}
-
 // The struct PrivateAccess is used to provide access to private members of
 // message classes without making them public. This is useful for highly
 // optimized code paths that need to access internals.