Remove a number of unused methods from Helper.

These methods were only ever used by FormHelper in the core. They are
complex and not useful not that FormHelper does not persist state.
I'm not aware of many userland Helpers that used this functionality
either, which makes their removal more straight-forward.
Allow the EntityContext to read from the request data.

Reading from the request data is helpful when you are making forms, as
you might need to access data that has not been added to the entities
that compose the context.
Optimize helper event listeners.

Most of the core helpers don't use event listeners. Stubbing out the
methods saves 6 calls to method_exists() per helper.
Fix failing tests.
Merge pull request #2958 from cakephp/3.0-form-cleanup

3.0 form cleanup