chore(client-cpp): trim examples README and formatDatetime dead code

Simplify the SDK layout note in examples README and drop unused-parameter
casts for precision/zoneId in formatDatetime.

Author: hongzhi-gao

iotdb-client/client-cpp/examples/README.md

@@ -69,7 +69,7 @@ client/
 ├── include/
 │   ├── Session.h
 │   ├── Export.h
-│   └── ...          (17 public headers; no thrift/ or boost/)
+│   └── ...
 └── lib/
     ├── iotdb_session.dll + iotdb_session.lib   (Windows)
     ├── libiotdb_session.so                     (Linux)

iotdb-client/client-cpp/src/session/Common.cpp

@@ -59,8 +59,6 @@ std::string getTimePrecision(int32_t timeFactor) {
 
 std::string formatDatetime(const std::string& format, const std::string& precision,
                            int64_t timestamp, const std::string& zoneId) {
-  (void)precision;
-  (void)zoneId;
   std::time_t time = static_cast<std::time_t>(timestamp);
   std::tm* tm = std::localtime(&time);
   char buffer[80];