Slope ascension was causing character to hover slightly

When ascending a slope sometimes you hover slightly above the ground,
and then when you stop moving you fall back down to the correct height.
This was caused by calculating the Y adjustment by total distance
traveled, instead of distance traveled while actually on the slope
itself.

This fix provides handleHorizontalSlope() with distanceToSlope, and only
calculates deltaMovement.y based on deltaMovement.x - distanceToSlope.
This excludes the approach to the slope on flat ground from the final
ascension value.

Also removed isGoingRight parameter since it is no longer used due to a
previous bug fix.