BHV-18024: Remove explicit directionality rule for Slider's popup.

### Issue
We inadvertently added a previously removed rule when making a fix for BHV-11836. This caused the negative sign for slider popup values to be displayed on the right side of the number in RTL locales.

### Fix
We remove this rule as `direction:ltr` is already set for the `moon-progress-bar` class, which `moon.Slider` inherits from via `moon.ProgressBar`. Additionally, we update the `SliderSample` to allow for easier testing of negative values.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>