fix UTFGreekAccents to remove superscript tilde

From DM Smith's bug report:

1) The Greek Accents --> Off does not remove all the accents. I am
using the default font on Win XP, which shows boxes for accented
characters. After turning them off, most are removed, but some boxes
remain. Comparing John 1.1 to my Greek text, nearly all the Greek
unaccented letters are present. but now some with boxes adjacent.