Revert one hunk of change 32034 that had the possibility of being buggy

Revert one hunk of change 32034 that had the possibility of being buggy
(the sprintf "%c" code will work correctly when the SV is UTF-8).
Audit all the rest for UTF-8 correctness, and force SvUTF-8_off() in
utf8.c to ensure correctness. (The string is reset to "", so this will
not be a behaviour change.)

p4raw-id: //depot/perl@32040