struct Rav1dFrameContextTaskThreadPendingTasks: remove unused code (#1452)

This seems to have been removed a long time ago in 8f78cfe, but we never
realized that it was dead code since it was `pub`. The newly released
Rust 1.89 warns about this, however, since it was never constructed
either, and our `cargo +stable clippy -- -D warnings` CI step uses the
latest stable.

Author: kkysen

src/internal.rs

@@ -664,13 +664,6 @@ pub struct Rav1dFrameContextLf {
     pub restore_planes: LrRestorePlanes,
 }
 
-#[derive(Default)]
-#[repr(C)]
-pub struct Rav1dFrameContextTaskThreadPendingTasks {
-    pub head: Rav1dTaskIndex,
-    pub tail: Rav1dTaskIndex,
-}
-
 #[derive(Default)]
 #[repr(C)]
 pub(crate) struct Rav1dFrameContextTaskThread {