Remove unused 'opaque' property from ol/source/Stamen options

And set the `layer` property as mandatory.

src/ol/source/Stamen.js

@@ -91,10 +91,9 @@ const ProviderConfig = {
 /**
  * @typedef {Object} Options
  * @property {number} [cacheSize=2048] Cache size.
- * @property {string} [layer] Layer.
+ * @property {string} layer Layer name.
  * @property {number} [minZoom] Minimum zoom.
  * @property {number} [maxZoom] Maximum zoom.
- * @property {boolean} [opaque] Whether the layer is opaque.
  * @property {number} [reprojectionErrorThreshold=0.5] Maximum allowed reprojection error (in pixels).
  * Higher values can increase reprojection performance, but decrease precision.
  * @property {import("../Tile.js").LoadFunction} [tileLoadFunction]