Revert "[NFC] Update new warning to test"

This reverts commit 25cdf87b.
125abb61 reverted the patch.

It is incorrect to update this file when a flagless warning is added.
This test exists to make sure _all_ warnings are behind a flag.
The right fix is to put the new warning in a warning group (so that
it can be toggled with a flag), not to update the list here.