river: remove wlroots 0.14 workarounds

Since updating to wlroots 0.15 we can remove several workarounds we had
in place due to wlroots improvements.

river/Cursor.zig

@@ -677,8 +677,6 @@ fn xwaylandUnmanagedSurfaceAt(lx: f64, ly: f64) ?SurfaceAtResult {
 }
 
 fn surfaceAtFilter(view: *View, filter_tags: u32) bool {
-    // TODO(wlroots): we can remove this view.surface != null check as surfaceAt
-    // will start filtering by mapped views by default in 0.15.0
     return view.surface != null and view.current.tags & filter_tags != 0;
 }