Remove leftover code in the DataContainer class

[fritzmg]

Fixes #4969

DataContainer::row uses $this->User->isAdmin on form submit - however at this stage the BackendUser instance is not imported yet.

[aschempp]

might also fix this by using security voters on ROLE_ADMIN 🙃

[fritzmg]

Yes, I just opted for the more minimal fix. I also wonder whether this has any unintended side-effects. Clearly this bit of code was never executed 😁

[fritzmg]

As discussed on Slack the adjustments to $newPaletteFields aren't actually necessary, so we can remove that and the $this->User->isAdmin check altogether.

[leofeyer]

Thank you @fritzmg.