Remove note on accepted invalid should_panic syntax

We had left a note about the fact that we accepted syntax for the
`should_panic` attribute that was invalid.  We no longer do, so let's
remove that note.

Author: traviscross

src/attributes/testing.md

@@ -161,9 +161,6 @@ The `should_panic` attribute has one of the following forms:
   > fn mytest() { panic!("error: some message, and more"); }
   > ```
 
-  > [!NOTE]
-  > `rustc` currently accepts the [MetaListNameValueStr] form with invalid syntax between the parentheses and emits a future-compatibility warning. This may become a hard error in the future.
-
 r[attributes.testing.should_panic.allowed-positions]
 The `should_panic` attribute may only be applied to functions annotated with the `test` attribute.