feat: OPTIC-184: Remove is_deleted field from User model and manager

[dredivaris]

This PR removes the is_delete field from the User model (as soft deletes are now handled by organization membership), as well as removing the model manager associated with filtering out soft deleted Users. As there are references to the the manager in the migrations, there was some trickiness involving removing the manager from old migrations (otherwise migrations run from the beginning would fail when trying to find removed manager.

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

(link to issue, supportive screenshots etc.)

What does this fix?

(if this is a bug fix)

What is the new behavior?

(if this is a breaking or feature change)

What is the current behavior?

(if this is a breaking or feature change)

What libraries were added/updated?

(list all with version changes)

Does this change affect performance?

(if so describe the impacts positive or negative)

Does this change affect security?

(if so describe the impacts positive or negative)

What alternative approaches were there?

(briefly list any if applicable)

What feature flags were used to cover this change?

(briefly list any if applicable)

Does this PR introduce a breaking change?

(check only one)

• Yes, and covered entirely by feature flag(s)
• Yes, and covered partially by feature flag(s)
• No
• Not sure (briefly explain the situation below)

What level of testing was included in the change?

(check all that apply)

• e2e
• integration
• unit

Which logical domain(s) does this change affect?

(for bug fixes/features, be as precise as possible. ex. Authentication, Annotation History, Review Stream etc.)

[netlify]

✅ Deploy Preview for label-studio-docs-new-theme canceled.

Name	Link
🔨 Latest commit	3a84764
🔍 Latest deploy log	https://app.netlify.com/sites/label-studio-docs-new-theme/deploys/656a3dd7881aec0008a1ced2

[netlify]

✅ Deploy Preview for heartex-docs canceled.

Name	Link
🔨 Latest commit	3a84764
🔍 Latest deploy log	https://app.netlify.com/sites/heartex-docs/deploys/656a3dd7698a0900089e3f42

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7731953) 76.11% compared to head (3a84764) 76.02%.
Report is 29 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5125      +/-   ##
===========================================
- Coverage    76.11%   76.02%   -0.09%     
===========================================
  Files          154      154              
  Lines        12792    12782      -10     
===========================================
- Hits          9736     9717      -19     
- Misses        3056     3065       +9     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.