Dead code cleanup
The following functions have declarations and definitions but are never actually called. Can they be removed?
_sion_cache_init
_sion_cache_destroy
sion_print_time_stamp
_sion_ompicol_size_of_dtype
_sion_set_fd
_sion_ptr2fd
_sion_fd_size
_sion_get_numfiles_from_file_mpi
_sion_get_filenumber_from_files_mpi
(_)sion_free_io_info
sion_set_fp_closed
(_)sion_set_second_fp
(_)sion_unset_second_fp
sion_optimize_fp_buffer
sion_flush
_sion_fileptrflags_to_str
sion_get_dfile
sion_isdebug
sion_debug_off
sion_generic_register_get_multi_filename_cb
_sion_free_filedesc_all_localranks