Re-enable the proc-macro repr(packed) ICE test under the parallel front-end

This test was marked ignore-parallel-frontend because the panic-time query
stack collection could nondeterministically print a "Failed to collect active
jobs" warning. With that warning lowered to debug! the ICE output is stable
across runs, so replace the directive with a blank line rather than deleting
it. The expected stderr is unchanged because the line numbers stay the same.

Author: xmakro

tests/ui/resolve/proc_macro_generated_packed.rs

@@ -1,6 +1,6 @@
 //! This test ICEs because the `repr(packed)` attribute
 //! was generated by a proc macro, so `#[derive]` didn't see it.
-//@ ignore-parallel-frontend failed to collect active jobs
+
 //@proc-macro: proc_macro_generate_packed.rs
 //@known-bug: #120873
 //@ failure-status: 101