Stop assigning users to Search Box experiment cohorts

[ayoy]

Task/Issue URL: https://app.asana.com/0/72649045549333/1208746673107049/f

Description:
This change stops assigning new users to experiment, while keeping Search Box visible for users that have previously been enrolled.

Steps to test this PR:
Run the following commands to make yourself enrolled into the Search Box experiment:

defaults write com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.cohort ntp_search_box_existing_user
defaults write com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.enrollment-date -date "2024-11-10 16:22:28 +0000"
defaults write com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.did-run-enrollment 1

Run the app and confirm that you see the search box.

Then call the following to reset the experiment:

defaults delete com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.did-run-enrollment
defaults delete com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.cohort
defaults delete com.duckduckgo.macos.browser.debug homepage.searchbox.experiment.enrollment-date

Then run the app and confirm that you don't see the search box.
Then verify that this command in terminal doesn't show any output:

defaults read com.duckduckgo.macos.browser.debug | grep homepage

Definition of Done:

• Does this PR satisfy our Definition of Done?

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