lisa.tests.starting.utilclamp: Fix for pandas 2.0.0

DataFrame.pivot(index=None) seems to have changed behavior:
pandas-dev/pandas#52436

Remove index=None so that it works across pandas versions.

Author: douglas-raillard-arm

lisa/tests/staging/utilclamp.py

@@ -172,7 +172,7 @@ def _get_trace_df(self):
 
         df_freq = self.trace.ana.frequency.df_cpus_frequency()
         df_freq = df_freq[['cpu', 'frequency']]
-        df_freq = df_freq.pivot(index=None, columns='cpu', values='frequency')
+        df_freq = df_freq.pivot(columns='cpu', values='frequency')
         df_freq.reset_index(inplace=True)
         df_freq.set_index('Time', inplace=True)