Remove repetitive call to get executor class from config

[khalidmammadov]

This is code improvement PR and removes call to get executor class from the config.
It will instead call self.executor property to invoke call to read from config and cache.
This also simplifies code.
I also added comment for that assignment as it looks like executor from the arguments only used for tests and not intended to be supplied by user.

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[kaxil]

This was intended, check #16700

[khalidmammadov]

I dont think this change will bring that issue back as executor assignment logic remains the same and loading executor will be the still the same. The only difference executor_class will be fetched either from executor which is just after that arguments assignment in the IF. Otherwise, it will be looked up through property.
Is there any test case that checks that scenario? Tests I run all passed.

[kaxil]

I dont think this change will bring that issue back as executor assignment logic remains the same and loading executor will be the still the same. The only difference executor_class will be fetched either from executor which is just after that arguments assignment in the IF. Otherwise, it will be looked up through property. Is there any test case that checks that scenario? Tests I run all passed.

As soon as you do self.executor_class = self.executor.__class__.__name__ (out of if statement) it needlessly loads the executor (check code block below) just to get the Class name. Please check detailed description in #16700

airflow/airflow/jobs/base_job.py

Lines 98 to 101 in 47f43ec

	if executor:
	# Executor here is used for test suites. So they can inject it for testing
	self.executor = executor
	self.executor_class = self.executor.__class__.__name__

airflow/airflow/jobs/base_job.py

Lines 111 to 112 in 47f43ec

	def executor(self):
	return ExecutorLoader.get_default_executor()