[landing] remove subscription form from footer

Summary:
Breaking down the redesign of the footer into two separate diffs. The first diff removes the subscription form and the related styles for it

Depends on D8081

Test Plan:
Please checkout the screenshot to see the subscription form being removed from the footer:

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8082