Fix platform_compatibility_test flakiness and re-enable it for TF1.

PiperOrigin-RevId: 462749230

Author: emilyfertig

tensorflow_probability/python/distributions/BUILD

@@ -4194,7 +4194,6 @@ py_test(
     shard_count = 50,
     tags = [
         "hypothesis",
-        "tf1-broken",  # TODO(b/239963967): Flaky under TF1.
     ],
     deps = [
         ":distributions",

tensorflow_probability/python/distributions/mvn_full_covariance.py

@@ -195,7 +195,6 @@ def __init__(self,
             allow_nan_stats=allow_nan_stats,
             name=name)
     self._parameters = parameters
-    self._covariance_matrix = covariance_matrix
 
   @classmethod
   def _maximum_likelihood_parameters(cls, value):