all: use trait upcasting

src/backends/metal.rs

@@ -52,7 +52,6 @@ use {
     },
     bstr::ByteSlice,
     std::{
-        any::Any,
         cell::{Cell, RefCell},
         error::Error,
         ffi::{CStr, CString},
@@ -233,10 +232,6 @@ impl Backend for MetalBackend {
         }
     }
 
-    fn into_any(self: Rc<Self>) -> Rc<dyn Any> {
-        self
-    }
-
     fn switch_to(&self, vtnr: u32) {
         self.session.switch_to(vtnr, move |res| {
             if let Err(e) = res {