Update freedm.dxy

doxygen 1.8.5 is complaining about obsolete options in freedm.dxy.
They're obscure and probably unnecessary for older doxygen, so let's
update the whole thing as it suggests.
Merge pull request #293 from scj7t4/doxygen

Update freedm.dxy
Remove unused local variable
Merge branch 'trivial/unused-variable' into protobuf_noprotobufs