Remove deprecated attribute from Navigation block

[tellthemachines]

Description

In the Navigation block included in the header, there is a setCascadingProperties attribute that is no longer in use. It was part of the initial implementation of flex layout in the Navigation block, but was removed before landing in core.
I only recently realised that, because the removal was already during the Beta stage, that attribute was included in the TT2 markup, and so must have been copied into TT3 and now into TT4 😅

It would be great if we could finally break the cycle! Removing the attribute will have no effect apart from cleaning up the markup, because that initial implementation was scrapped.