tests/eas/generic: Fix matplotlib import

I wasn't aware that
> import matplotlib.pyplot as plt
is different from
> from matplotlib import pyplot as plt

The former will not use the backend set in tests/__init__.py while
the latter will.

This commit also removes the fill_between() call that is used to draw
the expected_placement figure, as it seems the desired behaviour is
not supported with the Agg backend ("Unknown property step").

tests/eas/generic.py

@@ -28,7 +28,7 @@
 from trace import Trace
 from unittest import SkipTest
 
-import matplotlib.pyplot as plt
+from matplotlib import pyplot as plt
 import os
 
 WORKLOAD_PERIOD_MS =  16
@@ -306,7 +306,6 @@ def plot_expected_util(self, experiment, util_df):
                 color = 'red'
 
             tdf.plot(ax=ax[cpu], drawstyle='steps-post', title="CPU{}".format(cpu), color=color)
-            ax[cpu].fill_between(tdf.index, tdf, 0, step='post', color=color)
             ax[cpu].set_ylabel('Utilization')
 
             # Grey-out areas where utilization == 0