refactor: delete Int64ToString (#2130)

Author: PrajwalBorkar

src/codec/codec.h

@@ -19,7 +19,6 @@
 
 #include <map>
 #include <memory>
-#include <sstream>
 #include <string>
 #include <utility>
 #include <vector>
@@ -238,13 +237,6 @@ int32_t GetStrCol(int8_t* input, int32_t str_field_offset, int32_t next_str_fiel
                   int32_t type_id, int8_t* data);
 }  // namespace v1
 
-inline std::string Int64ToString(const int64_t key) {
-    std::stringstream ss;
-    ss << std::hex << key;
-    std::string key_str = ss.str();
-    return key_str;
-}
-
 }  // namespace codec
 }  // namespace openmldb