Delete offer_email_reset ff and "update email" at sign up functionality

What does this MR do and why?

Part (2/3) (backend cleanup) of this issue: #550449 (closed)

• Part (1) - frontend cleanup (will merge before this MR): !197411 (merged)
• Part (3) - will clean up db columns

This MR code that is no longer evoked now that the frontend has been deleted.

QA Instructions

100% honest, I'm entirely going off of the specs. I basically just went through these two MRs (!126345 (merged) and !194151 (merged).) and removed any code that seemed connected to the offer_email_reset feature flag or email_reset_offered_at user detail.

I am very sorry I don't have better QA instructions for this one.

Frontend

At http://127.0.0.1:3000/admin/users/{any username here} you should not see the "Email reset removed at:" field

Backend

The patch /users/update_email endpoint should no longer be available or exist