Remove unnecessary stuff from various Makefiles (Config Sanity, Pt 1)

* `WITH_UIP6` (the make variable, not the define) doesn't do anything. We remove it from example and regression makefiles to prevent confusion.
* With #322 in place, platform Makefiles no longer need to do `-DUIP_CONF_IPV6=1`. Makefile.include has done so for us.

I need someone to check [`platform/cooja/Makefile.cooja #L77`](https://github.com/contiki-os/contiki/blob/master/platform/cooja/Makefile.cooja#L77) and confirm that it can be removed as well. Fairly certain it can, but please someone check.