text: Title: Remove visualizer tests in type checking
  Body: Many of the type checking utilities, e.g. `is_classifier`, `is_regressor`, etc. have a note to remove lines of code that are unnecessary after #90 is implemented. 
  
  For example see: [yellowbrick/utils/types.py#L64](https://github.com/DistrictDataLabs/yellowbrick/blob/develop/yellowbrick/utils/types.py#L64):
  
  ```python
  def is_classifier(estimator):
      """
      Returns True if the given estimator is (probably) a classifier.
  
      Parameters
      ----------
      estimator : class or instance
          The object to test if it is a Scikit-Learn clusterer, especially a
          Scikit-Learn estimator or Yellowbrick visualizer
  
      See also
      --------
      is_classifier
          `sklearn.is_classifier() <https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/base.py#L518>`_
      """
      # TODO: once we make ScoreVisualizer and ModelVisualizer pass through
      # wrappers as in Issue #90, these three lines become unnecessary.
      # NOTE: This must be imported here to avoid recursive import.
      from yellowbrick.base import Visualizer
      if isinstance(estimator, Visualizer):
          return is_classifier(estimator.estimator)
  
      # Test the _estimator_type property
      return getattr(estimator, "_estimator_type", None) == "classifier"
  
  # Alias for closer name to isinstance and issubclass
  isclassifier = is_classifier
  ```
  
  We should remove these lines of code and **ensure the tests have correct coverage**. 