remove internal usage of redundant uci_ptr.last

remove internal usage of redundant uci_ptr.last

In uci_lookup_ptr and uci_set the pointer uci_ptr ptr.last is set to
the element corresponding to the first of: ptr.o, ptr.s, ptr.p.

Thus, ptr.last is redundant and in case of uci_set is (and was) not
always consistently set.

In order to simplify the code this commit removes internal usage
of ptr.last, and remove setting it from uci_set (and from uci_add_list
that was never used anyway).

As it is part of the public C api ptr.last cannot be completely
removed though. A search on lxr.openwrt.org shows that it is used as
the output of uci_lookup_ptr in several packages.

So we leave setting ptr.last in uci_lookup_ptr intact.

Signed-off-by: Jan Venekamp <jan@venekamp.net>