Remove xmac.c and lpp.c

This pull request is a proposal to remove the `xmac.c` and `lpp.c` radio duty cycling protocols. Neither of them are used by any of the platforms in the Contiki tree and there are no regression tests that test them.

The `xmac.c` code was once used as the default rdc protocol on several platforms, but is not replaced with `contikimac.c` and `cxmac.c`.

The `lpp.c` code was once an interesting experimental way to do radio duty cycling that eventually evolved into IEEE 802.15.4e RIT, but the `lpp.c` code particularly well-tested and there have been no commits to the code in 3+ years except for maintenance commits.

This pull request removes them so that we won't need to maintain code that isn't used and tested.