remove inaccurate comment in rabit.py

[jameslamb]

I was reading through the rabit source code today and saw this comment that looks out of place:

rabit/python/rabit.py

Line 46 in 4acdd7c

# load in xgboost library

I think it was probably accidentally copied over from xgboost.

[hcho3]

@jameslamb Hi James! Thanks for taking interest in Rabit. I just wanted to let you know that recent developments of Rabit have been moved into the XGBoost repository. See discussion in dmlc/xgboost#5995. We should probably archive this repository; unfortunately, I lack the necessary admin rights to archive it myself.

[jameslamb]

oh! Haha ok then you can close this and #146 . Thanks for pointing me to dmlc/xgboost#5995