[Enhancement] Optimize content scroll listening in modal

[RasmusKjeldgaard]

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Describe the enhancement

#2995 implements scroll-listening via the (ionScroll) event emitter on ion-content, for showing/hiding the horizontal divider below the title/header area on mobile.

Currently the listening is always enabled even though the divider line is always shown on desktop, no matter the scroll state of ion-content.

Describe the solution you'd like

Ideally we want to listen to the current breakpoint and enable/disable the subscription accordingly.

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