Remove useless character-length checks in contrib/ltree.

The t_iseq() macro does not need to be guarded by a character
length check (at least when the comparison value is an ASCII
character, as its documentation requires).  Some portions of
contrib/ltree hadn't read that memo, so simplify them.

The last change in gettoken_query,

-                else if (charlen == 1 && !t_iseq(state->buf, ' '))
+                else if (!t_iseq(state->buf, ' '))

looks like it's actually a bug fix: I doubt that the intention
was to silently ignore multibyte characters as if they were
whitespace.  I'm not tempted to back-patch though, because this
will have the effect of tightening what is allowed in ltxtquery
strings.

Discussion: https://postgr.es/m/2548310.1664999615@sss.pgh.pa.us

contrib/ltree/lquery_op.c

@@ -25,17 +25,16 @@ static char *
 getlexeme(char *start, char *end, int *len)
 {
 	char	   *ptr;
-	int			charlen;
 
-	while (start < end && (charlen = pg_mblen(start)) == 1 && t_iseq(start, '_'))
-		start += charlen;
+	while (start < end && t_iseq(start, '_'))
+		start += pg_mblen(start);
 
 	ptr = start;
 	if (ptr >= end)
 		return NULL;
 
-	while (ptr < end && !((charlen = pg_mblen(ptr)) == 1 && t_iseq(ptr, '_')))
-		ptr += charlen;
+	while (ptr < end && !t_iseq(ptr, '_'))
+		ptr += pg_mblen(ptr);
 
 	*len = ptr - start;
 	return start;