Fix broken asv (pandas-dev#29906)

Author: jbrockmendel

asv_bench/benchmarks/timeseries.py

@@ -113,7 +113,7 @@ class InferFreq:
     def setup(self, freq):
         if freq is None:
             self.idx = date_range(start="1/1/1700", freq="D", periods=10000)
-            self.idx.freq = None
+            self.idx._data._freq = None
         else:
             self.idx = date_range(start="1/1/1700", freq=freq, periods=10000)