Doxygen: disable CALL_GRAPH and CALLER_GRAPH for performance reasons

I was thinking that Doxygen should improve performance and remove unnecessary stuff. Also, bones_was_here mentioned in Darkplaces engine about that: "Many of these graphs are too huge and complex to be readable, and they use > 8GiB on disk and a lot of CPU time to generate. They can be enabled for specific funcs if required."

Moreover, shouldn't DOT_NUM_THREADS set to 0?