[web] [remove] background image css

Summary: was for background image, which we dont use anymore https://developer.mozilla.org/en-US/docs/Web/CSS/background-attachment https://developer.mozilla.org/en-US/docs/Web/CSS/background-size

Test Plan: n/a

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3111