tool: Support downcasting generic Ec

Removes the `impl Access for &mut dyn Access` I added earlier. But that
hasn't proven too useful.

tool/src/ec.rs

@@ -245,14 +245,6 @@ impl<A: Access> Ec<A> {
         self.command(Cmd::LedSetColor, &mut data)
     }
 
-    pub fn as_dyn(&mut self) -> Ec<&mut dyn Access> {
-        Ec {
-            access: &mut self.access,
-            version: self.version,
-
-        }
-    }
-
     pub fn into_dyn(self) -> Ec<Box<dyn Access>>
     where A: 'static {
         Ec {