Remove the DOMToTexture feature

This relies on WebRender's frame output API, `set_output_image_handler`,
which has been removed from the latest upstream [1]. It's sad to remove
this feature, which was probably a lot of work to implement, but it
seems difficult to patch WebRender to restore this functionality.

Fixes #29936.

1. https://hg.mozilla.org/mozilla-central/rev/361521e3c52324809553c555fb066d50f023d9bf

components/config/prefs.rs

@@ -259,11 +259,6 @@ mod gen {
                     #[serde(default)]
                     enabled: bool,
                 },
-                webgl: {
-                    dom_to_texture: {
-                        enabled: bool,
-                    }
-                },
                 webgl2: {
                     enabled: bool,
                 },