*Please note:* This only removes the preference itself, however both the viewer-option and the actual implementation is still available.
The `useOnlyCssZoom` functionality was only ever used, by default, in the PDF Viewer for the B2G/FirefoxOS project (which was abandoned years ago). Given that CSS-only zooming can easily make the document look blurry even at low zoom levels, this functionality was only intended for low-powered mobile devices.
Hence it seems reasonable to remove the `useOnlyCssZoom` preference now, since neither the default viewer nor the GeckoView-specific viewer uses this functionality.