Remove ambiguity sets (bevyengine#5916)

# Objective

Ambiguity sets are used to ignore system order ambiguities between groups of systems. However, they are not very useful: they are clunky, poorly integrated, and generally hampered by the difficulty using (or discovering) the ambiguity detector.

As a first step to the work in bevyengine#4299, we're removing them.

## Migration Guide

Ambiguity sets have been removed.

crates/bevy_ecs/macros/src/lib.rs

@@ -462,21 +462,6 @@ pub fn derive_stage_label(input: TokenStream) -> TokenStream {
     derive_label(input, &trait_path, "stage_label")
 }
 
-/// Generates an impl of the `AmbiguitySetLabel` trait.
-///
-/// This works only for unit structs, or enums with only unit variants.
-/// You may force a struct or variant to behave as if it were fieldless with `#[ambiguity_set_label(ignore_fields)]`.
-#[proc_macro_derive(AmbiguitySetLabel, attributes(ambiguity_set_label))]
-pub fn derive_ambiguity_set_label(input: TokenStream) -> TokenStream {
-    let input = parse_macro_input!(input as DeriveInput);
-    let mut trait_path = bevy_ecs_path();
-    trait_path.segments.push(format_ident!("schedule").into());
-    trait_path
-        .segments
-        .push(format_ident!("AmbiguitySetLabel").into());
-    derive_label(input, &trait_path, "ambiguity_set_label")
-}
-
 /// Generates an impl of the `RunCriteriaLabel` trait.
 ///
 /// This works only for unit structs, or enums with only unit variants.

crates/bevy_ecs/src/lib.rs

@@ -34,9 +34,9 @@ pub mod prelude {
         event::{EventReader, EventWriter, Events},
         query::{Added, AnyOf, ChangeTrackers, Changed, Or, QueryState, With, Without},
         schedule::{
-            AmbiguitySetLabel, ExclusiveSystemDescriptorCoercion, ParallelSystemDescriptorCoercion,
-            RunCriteria, RunCriteriaDescriptorCoercion, RunCriteriaLabel, Schedule, Stage,
-            StageLabel, State, SystemLabel, SystemSet, SystemStage,
+            ExclusiveSystemDescriptorCoercion, ParallelSystemDescriptorCoercion, RunCriteria,
+            RunCriteriaDescriptorCoercion, RunCriteriaLabel, Schedule, Stage, StageLabel, State,
+            SystemLabel, SystemSet, SystemStage,
         },
         system::{
             adapter as system_adapter, Commands, In, IntoChainSystem, IntoExclusiveSystem,

crates/bevy_ecs/src/schedule/label.rs

@@ -1,4 +1,4 @@
-pub use bevy_ecs_macros::{AmbiguitySetLabel, RunCriteriaLabel, StageLabel, SystemLabel};
+pub use bevy_ecs_macros::{RunCriteriaLabel, StageLabel, SystemLabel};
 use bevy_utils::define_label;
 
 define_label!(
@@ -13,12 +13,6 @@ define_label!(
     /// Strongly-typed identifier for a [`SystemLabel`].
     SystemLabelId,
 );
-define_label!(
-    /// A strongly-typed class of labels used to identify sets of systems with intentionally ambiguous execution order.
-    AmbiguitySetLabel,
-    /// Strongly-typed identifier for an [`AmbiguitySetLabel`].
-    AmbiguitySetLabelId,
-);
 define_label!(
     /// A strongly-typed class of labels used to identify [run criteria](crate::schedule::RunCriteria).
     RunCriteriaLabel,