Don't override global warnings filter (apache#12245)

Currently import mxnet causes warnings.filterwarnings('default', category=DeprecationWarning) to be executed.
This is very bad, as there are valid use cases where our users may have decided to filter out DeprecationWarning and we should not overwrite their configuration.

Author: leezu

python/mxnet/base.py

@@ -24,15 +24,12 @@
 import ctypes
 import os
 import sys
-import warnings
 import inspect
 import platform
 import numpy as np
 
 from . import libinfo
 
-warnings.filterwarnings('default', category=DeprecationWarning)
-
 __all__ = ['MXNetError']
 #----------------------------
 # library loading