Replace user provider system logic with SystemStore

[alexander-schranz]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
License	MIT

What's in this PR?

Replace user provider system logic with SystemStore.

Why?

This way create a custom listener to set a custom system for another user provider:

$this->systemStore->setSystem(YourAdmin::SYSTEM);