Remove unnecessary casts

Remove unnecessary casts

Some of these date back to pre-C89.
* src/buffer.c (backspace_output):
* src/create.c (to_base256, gid_to_chars, major_to_chars)
(minor_to_chars, off_to_chars, time_to_chars, uid_to_chars):
* src/list.c (from_header, tartime):
* src/map.c (owner_map_translate, group_map_translate):
* src/system.c (sys_truncate):
* src/utf8.c (utf8_init):
* src/xattrs.c (acls_one_line):
* src/xheader.c (xheader_string_end):
Remove casts.
* src/create.c (uintmax_to_chars): Remove.  All uses removed.
(simple_finish_header): Use to_octal.