chore(auth/types): remove undefined from forgot password mail types

[Sjoertjuh]

Description

This fixes a problem I had where the argument types of forgotPassword.generateEmailSubject and forgotPassword.generateEmailHTML can be undefined, which is not needed as the methods are never called without the arguments. This causes a lot of checks to be needed in a typescript project, which aren't really needed.

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Chore (non-breaking change which does not add functionality)

Checklist:

• [x] I have added tests that prove my fix is effective or that my feature works (none)
• [x] Existing test suite passes locally with my changes
• [x] I have made corresponding changes to the documentation (none)