Remove unnecessary PartialReflect bound on DeserializeWithRegistry (#17560)

# Objective

The new `DeserializeWithRegistry` trait in 0.15 was designed to be used
with reflection, and so has a `trait DeserializeWithRegistry:
PartialReflect` bound. However, this bound is not actually necessary for
the trait to function properly. And this `PartialReflect` bound already
exists on:
```rs
impl<T: PartialReflect + for<'de> DeserializeWithRegistry<'de>> FromType<T>
    for ReflectDeserializeWithRegistry
```
So there is no point in constraining the trait itself with this bound as
well.

This lets me use `DeserializeWithRegistry` with non-`Reflect` types,
which I want to do to avoid making a bunch of `FooDeserializer` structs
and `impl DeserializeSeed` on them.

## Solution

Removes this unnecessary bound.

## Testing

Trivial change, does not break compilation or `bevy_reflect` tests.

## Migration Guide

`DeserializeWithRegistry` types are no longer guaranteed to be
`PartialReflect` as well. If you were relying on this type bound, you
should add it to your own bounds manually.
```diff
- impl<T: DeserializeWithRegistry> Foo for T { .. }
+ impl<T: DeserializeWithRegistry + PartialReflect> Foo for T { .. }
```

crates/bevy_reflect/src/serde/de/deserialize_with_registry.rs

@@ -41,7 +41,7 @@ use serde::Deserializer;
 /// [`TypedReflectDeserializer`]: crate::serde::TypedReflectDeserializer
 /// [`ReflectDeserializer`]: crate::serde::ReflectDeserializer
 /// [via the registry]: TypeRegistry::register_type_data
-pub trait DeserializeWithRegistry<'de>: PartialReflect + Sized {
+pub trait DeserializeWithRegistry<'de>: Sized {
     fn deserialize<D>(deserializer: D, registry: &TypeRegistry) -> Result<Self, D::Error>
     where
         D: Deserializer<'de>;