remove unused import to fix pylint CI job (#3952)

Author: michaelosthege

pymc3/step_methods/metropolis.py

@@ -16,7 +16,6 @@
 import numpy.random as nr
 import theano
 import scipy.linalg
-import warnings
 
 from ..distributions import draw_values
 from .arraystep import ArrayStepShared, PopulationArrayStepShared, ArrayStep, metrop_select, Competence