patch 9.1.1574: Dead code in mbyte.c

patch 9.1.1574: Dead code in mbyte.c

Problem:  Dead code in mbyte.c
Solution: Delete the dead wcwidth()/iswprint() code
          (Damien Lejay)

These library calls have been disabled since patch 6.2.446 (2002) due to
display issues with Hebrew. They are also non-portable: wcwidth() is a
POSIX function and not available on MSVC or other non-POSIX platforms.
Keeping this code path adds complexity without benefit.

closes: #17811

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>