[landing] remove star background

Summary:
For the redesign of the landing page we are going to use a blackish background. The first step to achieve this is to remove the stars background which is what this diff does

Depends on D7756

Test Plan:
The two pages with the stars background animation (app-landing and keyserver) now have just a static background:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7757