[landing] Get rid of Request Access button and related stuff

Summary:
Get rid of the RequestAccess button in Heading (where it's visible for desktop breakpoints) and in HeroContent (where it's visible for mobile breakpoints).

Also got rid of the scrollToSubscriptionForm helper function

Leaving in the Button component, even though it's not used by anything else. Seems harmless to keep around and might be helpful in the future.

Test Plan:
Landing page continues to looks as expected.
Searched through repo pretty thoroughly to make sure I didn't leave behind any remnants of the button

How it looks on desktop:

How it looks on mobile (iPhone 13 mini):

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

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3402