Remove useless character-length checks in contrib/ltree.

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