clear useless code

db/db_impl.cc

@@ -1222,7 +1222,7 @@ std::vector DBImpl::FindKeysByField(Field &field) {
     std::string key = iter->key().ToString();
     Fields fields = iter->fields();
 
-    if (fields.find(field.first) == field.second) {
+    if (fields[field.first] == field.second) {
       keys.emplace_back(key);
     }
     iter->Next();

db/fields.cc

@@ -69,14 +69,4 @@ namespace leveldb {
 
     return field_array;
   }
-
-  std::string Fields::find(const std::string& field_name) const {
-    auto it = this->_fields.find(field_name);
-    if (it == this->_fields.end()) {
-      return "";
-    }
-    return it->second;
-  }
-
-
 }  // namespace leveldb

db/fields.h

@@ -33,8 +33,6 @@ namespace leveldb {
     // 将Fields编码为存入LevelDB的Value
     std::string Serialize() const;
 
-    // 通过field_name得到相应的字段值
-    std::string find(const std::string& field_name) const;
   private:
     std::map<std::string, std::string> _fields;
   };