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.

src/attributes/testing.md

diff --git a/src/attributes/testing.md b/src/attributes/testing.md
index 93fd7de..8fbcedd 100644
--- a/src/attributes/testing.md
+++ b/src/attributes/testing.md
@@ -161,9 +161,6 @@
   > 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.