macosx: Call VLCSliderCell methods directly in VLCSlider

Signed-off-by: Claudio Cambra <developer@claudiocambra.com>

modules/gui/macosx/views/VLCSlider.h

@@ -30,7 +30,4 @@
 /* Indicates if the slider is scrollable with the mouse or trackpad scrollwheel. */
 @property (readwrite) BOOL isScrollable;
 
-- (void)setSliderStyleLight;
-- (void)setSliderStyleDark;
-
 @end

modules/gui/macosx/views/VLCSlider.m

@@ -58,7 +58,7 @@ static inline void mouseEnteredOrExited(NSLayoutConstraint *const __unsafe_unret
         if (@available(macOS 10.14, *)) {
             [self viewDidChangeEffectiveAppearance];
         } else {
-            [self setSliderStyleLight];
+            [(VLCSliderCell*)self.cell setSliderStyleLight];
         }
         
         dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), ^(){
@@ -140,22 +140,12 @@ static inline void mouseEnteredOrExited(NSLayoutConstraint *const __unsafe_unret
     return NO;
 }
 
-- (void)setSliderStyleLight
-{
-    [(VLCSliderCell*)[self cell] setSliderStyleLight];
-}
-
-- (void)setSliderStyleDark
-{
-    [(VLCSliderCell*)[self cell] setSliderStyleDark];
-}
-
 - (void)viewDidChangeEffectiveAppearance
 {
     if (self.shouldShowDarkAppearance) {
-        [self setSliderStyleDark];
+        [(VLCSliderCell*)self.cell setSliderStyleDark];
     } else {
-        [self setSliderStyleLight];
+        [(VLCSliderCell*)self.cell setSliderStyleLight];
     }
 }