panic.rs: remove collapsible_if

[oech3]

No description provided.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[cakebaker]

Thanks!