Remove unused DEBUG macro about athena

[tomidakn]

I think it is OK to keep this option. However, the original policy of Athena++ is minimizing use of #ifdef. The problem of #ifdef is codes within the #ifdef - #endif block are not compiled and therefore not error checked (This is of course necessary for external library calls). Modern compilers should be able to optimize C++ if statements if they can be determined at the time of compilation. So I prefer to keep this policy, and encourage developers to use if instead of #ifdef as much as possible.

[tomidakn]

Oh but to do so, we need to change
#define DEBUG to #define DEBUG 1

[felker]

@tomidakn I agree that if (DEBUG) { ...} is highly preferred to #ifdef DEBUG. But I wonder if we shouldn't also be very strict about how many if (DEBUG) { ...} sections get merged to master. And also, the switch might make more sense to be decoupled from compiling with -g. Sometimes you will want the debug symbols in your executable without a bunch of your verbose DEBUG code running. Although I guess in that case you can manually add the --cflag=-g instead of -debug convenience flag for configure.py

[tomidakn]

This is a good point. Indeed, I agree that we should separate the debugger (-g) option and the DEBUG flag. Possibly, it may be better to have a separate DEBUG flag for each module. I also understand that it is better to reduce DEBUG codes as much as possible in the master.

I am not sure if this is the best yet, but one possibility is separating "debug" = less optimization and "verbose" = more diagnosis messages?