Layout 2020: Remove the floats preference

Enable floats always. It doesn't make sense to have this preference as
it's a basic feature of an experimental layout engine.

components/config/prefs.rs

@@ -445,9 +445,6 @@ mod gen {
                 flexbox: {
                     enabled: bool,
                 },
-                floats: {
-                    enabled: bool,
-                },
                 #[serde(default = "default_layout_threads")]
                 threads: i64,
                 viewport: {