maint: omit unnecessary to_uchar

maint: omit unnecessary to_uchar

* src/df.c (replace_control_chars):
* src/dircolors.c (parse_line):
* src/printf.c (print_esc):
* src/ptx.c (unescape_string):
* src/stat.c (print_it):
* src/tr.c (star_digits_closebracket):
Omit to_uchar calls that aren’t needed, because the parent
expression works with ‘char’ as well as with ‘unsigned char’.