Remove CrossEntropyResult and CrossEntropyResultDict (quantumlib#5687)

- Also remove the whole cross_entropy_benchmarking.py
    since those were the last exposed objects from it.
- Note that these were JSON serializable objects and the
    replacements are in reCirq.  Thus, we just deserialize to
    a dictionary of values instead.

Author: dstrain115

cirq-core/cirq/experiments/__init__.py

@@ -22,8 +22,6 @@
     two_qubit_state_tomography,
 )
 
-from cirq.experiments.cross_entropy_benchmarking import CrossEntropyResult, CrossEntropyResultDict
-
 from cirq.experiments.fidelity_estimation import (
     hog_score_xeb_fidelity_from_probabilities,
     linear_xeb_fidelity_from_probabilities,