Remove APP_TESTING feature flag

[AmitPhulera]

Product Description

No user-facing effects. The APP_TESTING feature flag (slug: app_testing) controlled visibility of an "Application Testing" link in the Applications tab dropdown menu. The flag was tagged TAG_DEPRECATED and is being removed per SAAS-19323.

The app_execution app itself remains intact and accessible to superusers/contractors via @require_superuser_or_contractor.

Technical Summary

• Removed the toggle-gated "Application Testing" dropdown menu entry from ApplicationsTab.dropdown_items in corehq/tabs/tabclasses.py
• Commented out the APP_TESTING toggle definition in corehq/toggles/__init__.py

Feature Flag

• Variable: APP_TESTING
• Slug: app_testing
• Tag: TAG_DEPRECATED
• Type: StaticToggle

Safety Assurance

Verified that this is not used in any environment and on production it is used on a test domain.

Safety story

This change removes a deprecated feature flag. The toggle evaluated to False by default, so removing it preserves the default behavior. The only effect was showing an "Application Testing" link in the Applications dropdown for domains with the flag enabled. The underlying app_execution views remain accessible to superusers and contractors independently of this toggle.

Automated test coverage

The corehq/tabs/tests/test_tabclasses.py tests pass with these changes. No tests existed that specifically tested the APP_TESTING toggle behavior.

QA Plan

Verify no domains rely on the flag:

cchq production django-manage list_ff_domains app_testing
cchq staging django-manage list_ff_domains app_testing
cchq india django-manage list_ff_domains app_testing
cchq eu django-manage list_ff_domains app_testing

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change