Fix wrong statement about system nodepool in create-node-pools.md

[JoeyC-Dev]

Proposed change: Fixing wrong concept of "system nodepool = control plane".

Supporting points:

1. Control Plane nodes or Pods in Control Plane is not visible to users. Reducing nodes in system nodepool will cause impact to control plane is false statement. Adding control plane nodes is also not an option for users.
2. Since it is never related to control plane, "lose control plane resources" is also false statement.

[prmerger-automator]

@JoeyC-Dev : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 5ada381:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/aks/create-node-pools.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[ttorble]

@schaffererin

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team