VideoPlayerViewController: Reduce brightness gesture to slider

This removes the old brightness gesture that could be started from the whole left side of the player.

With the introduction of the slider indicator for the brightness on the left, the user has a more interactive indicator for the brightness changes.

Indeed, this limits the user to start the brightness changes from the slider which is more UX friendly since the user directly interacts with the feature.

Note: More work needs to be done on the slider for a smoother/better UI.

Fix #988 (closed)