Revert "api: document preauthorized auth keys"

This reverts commit dd6472d4e8.

Reason: it appears I was just really really wrong or confused.

We added it to the old internal API used by the website instead,
not to the "v2" API.

Updates #2120
Updates #4571

Change-Id: I744a72b9193aafa7b526fd760add52148a377e83
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

api.md

@@ -317,14 +317,9 @@ Allows for updating properties on the device key.
 - Provide `false` to enable the device's key expiry. Sets the key to expire at the original expiry time prior to disabling. The key may already have expired. In that case, the device must be re-authenticated.
 - Empty value will not change the key expiry.
 
-`preauthorized`
-
-- If `true`, don't require machine authorization (if enabled on the tailnet)
-
 ```
 {
-  "keyExpiryDisabled": true,
-  "preauthorized": true
+  "keyExpiryDisabled": true
 }
 ```