Remove background colour palette for subtitles as the background colour cannot currently be modified whilst we only support the browser subtitle implementation.

Author: abeddow91

dotcom-rendering/src/paletteDeclarations.ts

@@ -7310,10 +7310,6 @@ const paletteColours = {
 		light: () => sourcePalette.neutral[86],
 		dark: () => sourcePalette.neutral[86],
 	},
-	'--loop-video-subtitle-background': {
-		light: () => transparentColour(sourcePalette.neutral[7], 0.7),
-		dark: () => transparentColour(sourcePalette.neutral[7], 0.7),
-	},
 	'--loop-video-subtitle-text': {
 		light: () => sourcePalette.neutral[100],
 		dark: () => sourcePalette.neutral[100],