Remove unnecessary isolation_level setting from load_defaults 7.0

[r7kamura]

Motivation / Background

The default value of ActiveSupport::IsolatedExecutionState.isolation_level is set to :thread, regardless of whether load_defaults 7.0 is used or not.

• https://github.com/rails/rails/blob/v7.0.4/activesupport/lib/active_support/isolated_execution_state.rb

If so, there is no need to set this in load_defaults 7.0. This Pull Request has been created to remove this unnecessary setting.

Or if there is any reason to set this in load_defaults 7.0, I would like to know that.

Detail

This Pull Request simply removes this setting.

Additional information

I confirmed that ActiveSupport::IsolatedExecutionState.isolation_level returns :thread on Rails 7.0.4 with load_defaults 6.1.

$ bin/rails c -e test
Loading test environment (Rails 7.0.4)
irb: warn: can't alias context from irb_context.
irb(main):001:0> ActiveSupport::IsolatedExecutionState.isolation_level
=> :thread
irb(main):002:0> exit

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• CI is passing.

[skipkayhil]

If this is removed we should clean up the docs as well:

rails/guides/source/configuring.md

Line 99 in de9f971

- [`config.active_support.isolation_level`](#config-active-support-isolation-level): `:thread`

and if backported to 7-0-stable it should probably be removed from new_framework_defaults_7_0:

rails/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_0.rb.tt

Lines 43 to 46 in a4dbb15

	# Define the isolation level of most of Rails internal state.
	# If you use a fiber based server or job processor, you should set it to `:fiber`.
	# Otherwise the default of `:thread` if preferable.
	# Rails.application.config.active_support.isolation_level = :thread

[r7kamura]

Good point. I agree that and I will make that change in this Pull Request, at least for guides/source/configuring.md.

[skipkayhil]

@r7kamura Would you mind rebasing? I'll see if I can get it a review