Delete TargetTransformer class

[freddyaboulton]

The TargetTransformer parent class was added when the pipeline/component graph inverse_transform method was added. We felt it was necessary to introduce this parent class because our component graph was not 100% ready to handle components that modified the target, e.g. we still supported from_list for the component graph even though lists are an ambiguous representation for pipelines with target transformers and it was possible to grab the .y from a component that did not modify the target.

You can see this thread from the original PR for context: #2256 (comment)

The TargetTransformer class is no longer necessary because of:
#2563
#2556
#2662