Removed ResourceVariable._cached_shape_as_list

_shape_as_list is no longer on the hotpath of EagerLinearRegressionBenchmark,
the change does not seem to affect throughput estimates. Note that both
estimates have high variance.

Before:

    entry {
      name: "EagerLinearRegressionBenchmark.eager_train_cpu"
      iters: 2000
      wall_time: 1.24479007721
      extras {
        key: "examples_per_sec"
        value {
          double_value: 102828.583183
        }
      }
    }

After:

    entry {
      name: "EagerLinearRegressionBenchmark.eager_train_cpu"
      iters: 2000
      wall_time: 1.25307798386
      extras {
        key: "examples_per_sec"
        value {
          double_value: 102148.470924
        }
      }
    }

PiperOrigin-RevId: 228343127