drop feature importance outputs for estimators that do not calculate importance scores

[nbokulich]

Improvement Description
Now that we have TypeMap... A few estimators do not actually report feature importances, so let's no longer have them output fake feature importance scores. See this TODO:

q2-sample-classifier/q2_sample_classifier/utilities.py

Lines 347 to 350 in 9526961

	# TODO: drop this when we get around to supporting optional outputs
	# methods cannot output an empty importances artifact; only KNN has no
	# feature importance, but just warn and output all features as
	# importance = 1

Current Behavior
For knn (and any other estimators that do not calculate importance), importance = 1 for all features, and we issue a warning that these are fake.

Proposed Behavior
No longer output these scores.