Remove commented-out code (#941)

Not sure why this was commented out, but [a Code Quality scan flagged
it](https://github.com/quantumlib/qsim/security/quality/rules/py%2Fcommented-out-code),
so we may as well remove it.

Author: mhucka

qsimcirq_tests/qsimcirq_test.py

@@ -26,9 +26,6 @@ class NoiseTrigger(cirq.Gate):
     new circuit will otherwise behave identically to the original.
     """
 
-    # def _mixture_(self):
-    #   return ((1.0, np.asarray([1, 0, 0, 1])),)
-
     def _num_qubits_(self) -> int:
         return 1