bevy_reflect: Remove PartialReflect::serializable (#16576)

# Objective

`PartialReflect::serializable` is unused in the codebase and should be
removed.

I believe it originally was used to handle serializing certain types but
that's no longer the case.

## Solution

Remove `PartialReflect::serializable`.

## Testing

You can check locally using:

```
cargo check -p bevy_reflect --all-features
```

---

## Migration Guide

`PartialReflect::serializable` has been removed. If you were using this
to pass on serialization information, use `ReflectSerialize` instead or
create custom type data to generate the `Serializable`.

crates/bevy_reflect/src/func/dynamic_function.rs

@@ -8,7 +8,6 @@ use crate::{
         DynamicFunctionMut, Function, FunctionOverloadError, FunctionResult, IntoFunction,
         IntoFunctionMut,
     },
-    serde::Serializable,
     ApplyError, MaybeTyped, PartialReflect, Reflect, ReflectKind, ReflectMut, ReflectOwned,
     ReflectRef, TypeInfo, TypePath,
 };
@@ -428,10 +427,6 @@ impl PartialReflect for DynamicFunction<'static> {
         Debug::fmt(self, f)
     }
 
-    fn serializable(&self) -> Option<Serializable> {
-        None
-    }
-
     fn is_dynamic(&self) -> bool {
         true
     }

crates/bevy_reflect/src/reflect.rs

@@ -1,6 +1,6 @@
 use crate::{
-    array_debug, enum_debug, list_debug, map_debug, serde::Serializable, set_debug, struct_debug,
+    array_debug, enum_debug, list_debug, map_debug, set_debug, struct_debug, tuple_debug,
-    tuple_debug, tuple_struct_debug, DynamicTypePath, DynamicTyped, OpaqueInfo, ReflectKind,
+    tuple_struct_debug, DynamicTypePath, DynamicTyped, OpaqueInfo, ReflectKind,
     ReflectKindMismatchError, ReflectMut, ReflectOwned, ReflectRef, TypeInfo, TypePath, Typed,
 };
 use alloc::boxed::Box;
@@ -270,13 +270,6 @@ where
         }
     }
 
-    /// Returns a serializable version of the value.
-    ///
-    /// If the underlying type does not support serialization, returns `None`.
-    fn serializable(&self) -> Option<Serializable> {
-        None
-    }
-
     /// Indicates whether or not this type is a _dynamic_ type.
     ///
     /// Dynamic types include the ones built-in to this [crate],