Local navigation bar block: Remove default background color property.

The custom color is set using `style.color.background`, not `backgroundColor`. So when using a custom color, technically there is no `backgroundColor` attribute. The change to correctly inherit default attribute values was then setting a backgroundColor, which the color block-support prioritizes when building the style code, overriding the custom color.

This default can be removed, the CSS applies the blueberry color if no other colors are set.

Fixes #654

mu-plugins/blocks/local-navigation-bar/src/block.json

@@ -13,8 +13,7 @@
 			"default": "full"
 		},
 		"backgroundColor": {
-			"type": "string",
-			"default": "blueberry-1"
+			"type": "string"
 		},
 		"layout": {
 			"type": "object",