decoder: remove take_free_frame method from FramePool

The backend is the only one requesting frames from the pool, and it has
access to its exact type so it can call the type-specific method there.

This actually highlights a design issue with our frame pool and a need
for redesign.

src/backend/dummy/decoder.rs

@@ -121,10 +121,6 @@ impl<M> FramePool<M> for Backend {
     }
 
     fn clear(&mut self) {}
-
-    fn take_free_frame(&mut self) -> Option<Box<dyn AsRef<M>>> {
-        None
-    }
 }
 
 impl<Codec: StatelessCodec> StatelessDecoderBackendPicture<Codec> for Backend {

src/backend/vaapi/surface_pool.rs

@@ -235,11 +235,4 @@ impl<M: SurfaceMemoryDescriptor> FramePool<M> for VaSurfacePool<M> {
         pool.surfaces.clear();
         pool.managed_surfaces.clear();
     }
-
-    fn take_free_frame(&mut self) -> Option<Box<dyn AsRef<M>>> {
-        (**self)
-            .borrow_mut()
-            .get_surface(self)
-            .map(|s| Box::new(s) as Box<dyn AsRef<M>>)
-    }
 }

src/decoder.rs

@@ -43,14 +43,6 @@ pub trait FramePool<M> {
     fn num_managed_frames(&self) -> usize;
     /// Remove all frames from this pool.
     fn clear(&mut self);
-    /// Returns an object holding one of the available frames from this pool.
-    /// The frame will be available for rendering again once the returned object
-    /// is dropped.
-    ///
-    /// This is useful to prevent decoding from happening by holding all the available frames.
-    ///
-    /// Returns `None` if there is no free frame at the time of calling.
-    fn take_free_frame(&mut self) -> Option<Box<dyn AsRef<M>>>;
 }
 
 /// Information about the current stream.