Remove 'secret' option
|Reported by:||nacin||Owned by:||westi|
We no longer use the 'secret' option, which held an early security salt we no longer use.
It was added in 2.0.3 and lengthened in 2.5.1, and we stopped using it in 2.7. It isn't referenced anywhere, except in schema.php and upgrade.php.
I would have dropped it as part of #12159, but since it was used in a function in pluggable.php, then it may hypothetically still be in use. Do we yank it?