E2E: Remove Chemlab GitLab Login Page

What does this MR do and why?

Relates to GitLab Fulfillment E2E Tests: Phase out `data-q... (gitlab-org/quality/quality-engineering/team-tasks#1931 - closed)

This MR removes the Chemlab GitLab::Page::Main::Login page and its associated stub file, as it is not currently being used by any GitLab E2E tests.

The data-qa-selectors associated with the elements in this file are still in use by several other E2E tests (ex: password_confirmation_field, which is also defined and used here). Migrating these to data-testids is outside the scope of this MR and sectionfulfillment

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.