Do not show selection warning

Summary:
Do not show the visibility warning for the MxN Display

For decoupled render windows (i.e. MxN Display) the warning should not be shown.
Since only the global visibility-property is checked here we do not now if a
renderer-specific visibility is set to true and the reference- and
working-node are visible in a decoupled / desynchronized render window.

Test Plan:
manual testing

• open MITK, load an image
• close the Standard Display, open the MxN Display, create two render windows
• open the Segmentation view, create a new segmentation node
• decouple / unlock one render window (lock symbol)
• hide / deselect the reference / working image node in the still locked render window
• see how the segmentation tools are not disabled

Reviewers: s434n, O1 MITK Reviewer Group I, kislinsk

Reviewed By: s434n, O1 MITK Reviewer Group I, kislinsk

Subscribers: s434n, kalali

Maniphest Tasks: T29529

Differential Revision: https://phabricator.mitk.org/D796