Remove `unsafe impl {Send,Sync} for {Rav1dContext,TaskThreadData_delayed_fg}`

[kkysen]

These were TODOs and are unjustified/unsound.

The types that need to be Send + Sync are:

• Because of raw ptrs:
  • CArc<T>::stable_ref
  • Rav1dPicAllocator::cookie
  • Rav1dPictureData::allocator_data
  • Dav1dITUTT35::payload
• Because of UnsafeCell:
  • DisjointMut<T>::inner

For these:

• Rav1dPicAllocator::cookie
• Rav1dPictureData::allocator_data
• Dav1dITUTT35::payload

which are for C APIs, we should just delegate the safety requirements to # Safety docs. A wrapper Send + Sync ptr type might be useful.

For these:

• CArc<T>::stable_ref
• DisjointMut<T>::inner

Send + Sync is justified by the type and its API.