[server] Remove unused variable

This change suppresses the following warning:

    HostResourceQueryOption.cc: In member function 'virtual std::string HostResourceQueryOption::getFromClauseWithHostgroup() const':
    HostResourceQueryOption.cc:451:19: warning: unused variable 'hgrpColumnDefs' [-Wunused-variable]
      const ColumnDef *hgrpColumnDefs =
                       ^