Simplify the syntax for macros generated by rustc_queries

- Disallow multiple macros callbacks in the same invocation. In practice, this was never used.
- Remove the `[]` brackets around the macro name
- Require an `ident`, not an arbitrary `tt`

compiler/rustc_query_impl/src/profiling_support.rs

@@ -320,5 +320,5 @@ pub fn alloc_self_profile_query_strings(tcx: TyCtxt<'_>) {
         }
     }
 
-    rustc_query_append! { [alloc_once!] }
+    rustc_query_append! { alloc_once! }
 }