shellquoting: rm redundant iswprint() call (re: f9d28935)

A regression test failure was occurring on FreeBSD for
  bin/shtests -u builtins
because UTF-8 characters were wrongly encoded as bytes in the
C.UTF-8 locale. The cause is that iswprint() always returns false
on FreeBSD if the ksh-specific C.UTF-8 locale is active, as the OS
doesn't support it.

That iswprint() call is redundant anyway; the new is_invisible()
function now handles this.

src/cmd/ksh93/sh/string.c: sh_fmtq():
- Remove redundant iswprint() test.

src/cmd/ksh93/sh/string.c

@@ -459,7 +459,7 @@ char	*sh_fmtq(const char *string)
 					sfprintf(staksp,"\\u[%x]",c);
 					continue;
 				}
-				else if(!iswprint(c) || isbyte)
+				else if(isbyte)
 #else
 				if(!isprint(c))
 #endif