lib/vsprintf.c: remove unused ip6_addr_string()

There's currently no user of %p[iI]6, so including ip6_addr_string()
in the image is a waste of bytes. It's easy enough to have the
compiler elide it without removing the code completely.

The closest I can find to anybody "handling" ipv6 in U-Boot currently
is in efi_net.c which does

        if (ipv6) {
                ret = EFI_UNSUPPORTED;

As indicated in the comment, it can easily be put back, but preferably
under a config knob.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>