Remove providers imports from core examples

[turbaszek]

Core example DAGs should not depend on any non-core dependency
like providers packages.

I also added a pre-commit check to avoid regression:

➜ pre-commit run no-providers-in-core-examples --all-files
No providers imports in core example DAGs................................Failed
- hook id: no-providers-in-core-examples
- exit code: 1

airflow/example_dags/example_dag_decorator.py:25:from airflow.providers.http.operators.http import SimpleHttpOperator

closes: #12247

---

^ Add meaningful description above

Read the Pull Request Guidelines for more information.
In case of fundamental code change, 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 UPDATING.md.

[turbaszek]

Removed this as it was duplicated:

airflow/.pre-commit-config.yaml

Lines 397 to 402 in 3ddf4d0

	- id: pre-commit-descriptions
	name: Check if pre-commits are described
	entry: ./scripts/ci/pre_commit/pre_commit_check_pre_commits.sh
	language: system
	files: ^.pre-commit-config.yaml$|^STATIC_CODE_CHECKS.rst|^breeze-complete$
	require_serial: true

airflow/.pre-commit-config.yaml

Lines 459 to 464 in 3ddf4d0

	- id: pre-commit-descriptions
	name: Check if pre-commits are described
	entry: ./scripts/ci/pre_commit/pre_commit_check_pre_commits.sh
	language: system
	files: ^.pre-commit-config.yaml$|^STATIC_CODE_CHECKS.rst|^breeze-complete$
	require_serial: true

[turbaszek]

@mik-laj @potiuk is there any particular reason why http package is released as provider? It uses only

import requests
import tenacity

which are always installed with Airflow 🤔

[turbaszek]

I hope no one will be angry that I added this in this PR 😄

[mik-laj]

@turbaszek There is no reason. I've tried to move providers as many as possible, but we can move it back to the core.

[github-actions]

The PR needs to run all tests because it modifies core of Airflow! Please rebase it to latest master or ask committer to re-run it!

[kaxil]

Nice one @turbaszek ✅