Remove deprecated hook code from plugins

[kaxil]

We had removed registering hooks via plugin in #12108 for Airflow 2.0. We kept it for registering Connection form.

As I understand it, we don't use that code but ProvidersManager (airflow/providers_manager.py) to register Connections from providers.

Our docs already mark this as removed in 2.0:

airflow/docs/apache-airflow/authoring-and-scheduling/plugins.rst

Lines 33 to 37 in 1568788

	.. versionchanged:: 2.0
	Importing operators, sensors, hooks added in plugins via
	``airflow.{operators,sensors,hooks}.<plugin_name>`` is no longer supported, and these extensions should
	just be imported as regular python modules. For more information, see: :doc:`../administration-and-deployment/modules_management` and
	:doc:`/howto/custom-operator`

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[kaxil]

@ashb since you were the author of #12108 , could you review this PR and see if it makes sense

[potiuk]

Should be fine. The plugins from Airlfow 2 will stop working anyway and people will have to modify them. This is pretty much a "given" incompatibility and we need to bite the bullet. But the test needs to be fixed and I think we should add a newsfragment (just one liner now and we can expand it later).

[kaxil]

Should be fine. The plugins from Airlfow 2 will stop working anyway and people will have to modify them. This is pretty much a "given" incompatibility and we need to bite the bullet. But the test needs to be fixed and I think we should add a newsfragment (just one liner now and we can expand it later).

That was already part of the docs though, check PR description -- should we still add a newsfragment?

Our docs already mark this as removed in 2.0:

airflow/docs/apache-airflow/authoring-and-scheduling/plugins.rst

Lines 33 to 37 in 1568788

	.. versionchanged:: 2.0
	Importing operators, sensors, hooks added in plugins via
	``airflow.{operators,sensors,hooks}.<plugin_name>`` is no longer supported, and these extensions should
	just be imported as regular python modules. For more information, see: :doc:`../administration-and-deployment/modules_management` and
	:doc:`/howto/custom-operator`

[ashb]

Yeah looks good to me.

I think an explicit news fragment is a good idea, even though the docs say it was deprecated this should be called out as finally breaking. (I'm thinking to the issue FAB/WTForms had recently, so lets be explicit when we can)

[kaxil]

Added in 3f92bf7