ENYO-4048: Allow for overriding of scroller options with falsy values.

Issue
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It is not currently possible to override scroller properties with falsy values (i.e. properties that are true by default, such as `useMouseWheel`).

Fix
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Removed the `exists` option when mixing in `scrollerOptions` as there did not appear to be a current need for this (and there is no relevant history for the existence of this option here).

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>