Clean up `#[transactional]` (#11546)

* Deprecate #[transactional] attribute

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Remove #[transactional] from nomination pools

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Review fix

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix NOOP test

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Suppress warnings

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>