time_zone: Don't double up the std::abs

Too many absolute values were causing mirrored time zones to resolve
as the same.

src/common/time_zone.cpp

@@ -100,8 +100,7 @@ std::string FindSystemTimeZone() {
         }
 
         const auto offset = offsets[i];
-        const int difference =
+        const int difference = static_cast<int>(std::abs(offset - system_offset));
-            static_cast<int>(std::abs(std::abs(offset) - std::abs(system_offset)));
         if (difference < min) {
             min = difference;
             min_index = i;