[landing] Remove images that are no longer used

Summary:
These images were from a previous iteration of the landing page (the vertically paginated scroll-snap one) and are no longer used (did a search of the repo).

The images we currently have on the site live exclusively on S3 and aren't checked into the repo. We might want to do that? (cc @benschac)

Test Plan: Searched through the repo for occurences of these images, and couldn't find any.

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

Reviewed By: benschac, palys-swm

Subscribers: ashoat, Adrian, benschac, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3443