ref(flags): Remove organizations:new-organization-member-invite

[wedamija]

The flag scanner classified this as flagpole-disabled. The flag gated the entire invited-members API surface — index, details, and reinvite endpoints all returned 403 MISSING_FEATURE when False. Removes the never-reachable endpoints (but keeps the OrganizationMemberInvite model and its serializers), their URL routes, and the corresponding tests.

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.40%

[github-actions]

Backend Test Failures

Failures on 9fb7a94 in this run:

tests/sentry/test_mypy_stronglist.py::test_stronglist — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/test_mypy_stronglist.py:20: in test_stronglist
    raise AssertionError(f"\n\n{proc.stdout}")
E   AssertionError: 
E   
E   pyproject.toml: tests.sentry.api.endpoints.test_organization_member_reinvite in weaklist does not match any files!