jetpack: Slideshow: Fix doubled navigation arrows (#47262)

It looks like Slider at some point changed from settings its navigation
arrows using CSS `::after` to embedding an SVG in the DOM. Update the
block's CSS to hide that SVG where we're supplying our own via CSS
`background-image`.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/22300127656

Upstream-Ref: Automattic/jetpack@5b09957

Author: gmjuhasz