[PM-3833] Remove deprecated register and prelogin from API

[Hinton]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Remove the deprecated register and prelogin endpoints from the API service. I also moved the request and responses into the identity project and removed the duplicated test.

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• If making database changes - make sure you also update Entity Framework queries and/or migrations
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – 964b88f9-8e65-440a-a49e-ffb972d8d537

Fixed Issues

Severity	Issue	Source File / Package	Checkmarx Insight
	Privacy_Violation	/src/Core/Services/Implementations/UserService.cs: 323	Attack Vector

[JaredSnider-Bitwarden]

Closing this in favor of #4206.