[web] Remove div.devices and associated assets from Splash component

Summary:
Here's what it looks like after:

Test Plan:
Searched through repo for occurences of "android_screenshot" and "ios_screenshot" and couldn't find any.

Searched through repo for occurences of "css.devices" and ".devices" and couldn't find anywhere else in the codebase where those styles were used.

Made sure removing the div didn't cause any layout issues (though the whole thing is going anyways so it probably doesn't matter).

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

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3445