Stop pylint complaining about useless import alias. (#2655)

Recent pylint warngs about import renames with no effect.  Remove
them.

python/tvm/rpc/client.py

@@ -9,7 +9,7 @@ import time
 from . import base
 from ..contrib import util
 from .._ffi.base import TVMError
-from .._ffi import function as function
+from .._ffi import function
 from .._ffi import ndarray as nd
 from ..module import load as _load_module
 

vta/python/vta/ir_pass.py

@@ -3,7 +3,7 @@
 from __future__ import absolute_import as _abs
 
 import tvm
-from topi import util as util
+from topi import util
 
 from .environment import get_env