Fix encoding ItemStack in TrackedData (valence-rs#541)

When encoding `ItemStack`s in `TrackedData` the `ItemStack` itself is
not optional as it was previously implemented. For further context, when
encoding an `ItemStack` it will only encode as one byte if the
`ItemStack` is empty.

# Objective

- Fixes valence-rs#537 

# Solution

- Remove the optional wrapping around the `ItemStack` value when
encoding it in the `TrackedData`.

crates/valence_entity/build.rs

@@ -262,7 +262,6 @@ impl Value {
         match self {
             Value::Integer(_) => quote!(VarInt(#self_lvalue)),
             Value::OptionalInt(_) => quote!(OptionalInt(#self_lvalue)),
-            Value::ItemStack(_) => quote!(Some(&#self_lvalue)),
             _ => quote!(&#self_lvalue),
         }
     }