display: do not show a "[" double-width placeholder when softwrapping

The placeholder half-duplicates the character that is actually shown
at the beginning of the next row.  When NOT softwrapping, showing a
"[" before the ">" kind of makes sense: to avoid having a gaping gap
there.  But when softwrapping, all characters of the line will be on
the screen (except when going offscreen at the bottom), so there is
no need to show a placeholder for anything.

I imagine that when reading softwrapped Chinese or Japanese text,
things are easier to understand when no characters are shown that
are not actually in the text.

This avoids https://savannah.gnu.org/bugs/?57993.

Bug existed since version 2.8.6, commit 8490f4ac.

src/winio.c

@@ -2860,11 +2860,6 @@ int update_softwrapped_line(linestruct *line)
 		if (end_of_line)
 			break;
 
-		/* If the line is softwrapped early (because of a two-column character),
-		 * show a "[" placeholder, unless we're softwrapping at blanks. */
-		if (!ISSET(AT_BLANKS) && to_col - from_col < editwincols)
-			mvwaddch(edit, row - 1, to_col - from_col, '[');
-
 		from_col = to_col;
 	}