This PR removes the JSON-version of signup API handler, and its unused
references. We use the other Form-body-version of this API, but not
this. So this isn't needed.
Fewer things there are, fewer there are to protect.
## Automation
/test sanity authentication
### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results -->
> [!TIP]
> 🟢🟢🟢 All cypress tests have passed! 🎉🎉🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/11838832000>
> Commit: f27d2c9fa7d9b154eb6649b1f9885fd54d1921b5
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=11838832000&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.Sanity, @tag.Authentication`
> Spec:
> <hr>Thu, 14 Nov 2024 14:49:54 UTC
<!-- end of auto-generated comment: Cypress test results -->
## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Introduced new Cypress commands for enhanced API interactions and UI
validations.
- Added a method for handling user creation through form-encoded data.
- **Bug Fixes**
- Improved error handling and validation checks in various commands.
- **Documentation**
- Updated type definitions and submit handler functions for better
clarity and maintainability.
- **Chores**
- Removed unused user management functionalities, streamlining the
codebase.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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