[web] Remove "Request Access" markup/styling from Splash component

Summary:
Remove all JSX/CSS related to legacy "Request Access" functionality from the Splash component.

Note: There's still stray functionality/types/etc. in other files which will be dealt with in a separate diff. This diff focuses exclusively on splash.react.js and splash.css.

depends on D3446

Test Plan:
The only thing we really care about keeping around is the LogInModal.

1. Click "Log in" in the header
2. Enter credentials
3. Make sure user is properly authenticated and redirected to web app

Reviewers: def-au1t, palys-swm, varun, benschac

Reviewed By: palys-swm, benschac

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3447