Remove ItemProperty::Unsupported

Since read_ipco now returns a index -> property map rather than a vector of
properties, there's no longer a need to have a placeholder variant for the
indices of properties we don't support.

Author: baumanj

mp4parse/src/lib.rs

@@ -1612,7 +1612,6 @@ fn read_iprp<T: Read>(src: &mut BMFFBox<T>) -> Result<TryVec<AssociatedProperty>
 pub enum ItemProperty {
     Channels(TryVec<u8>),
     AuxiliaryType(AuxiliaryTypeProperty),
-    Unsupported,
 }
 
 /// For storing ItemPropertyAssociation data