Move associated type bounds check to ast lowering

This makes the check for when associated type bounds more accurate

compiler/rustc_ast_passes/src/errors.rs

@@ -23,13 +23,6 @@ pub struct ForbiddenLetStable {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_forbidden_assoc_constraint)]
-pub struct ForbiddenAssocConstraint {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_keyword_lifetime)]
 pub struct KeywordLifetime {