Remove unused CryptoCallbacks implementations (#12919)

* Remove unused `onSecretRequested` callback

This thing is unused with the rust crypto stack (which is lucky, because it
uses methods that only work with the legacy stack).

* Remove unused `getDehydrationKey` method

This callback is no longer used, so there is no need for an implementation.

* Remove unused `dehydrationCache`

This is no longer written to, so is redundant.

* Remove another write to `CryptoCallbacks.getDehydrationKey`

As before: this hook is no longer used by the js-sdk, so writing to it is
pointless.