Simplify default export

This removes unnecessary variable declarations where the default object is an object.

src/ol/extent/Corner.js

@@ -1,6 +1,7 @@
 /**
  * @module ol/extent/Corner
  */
+
 /**
  * Extent corner.
  * @enum {string}

src/ol/extent/Relationship.js

@@ -1,6 +1,7 @@
 /**
  * @module ol/extent/Relationship
  */
+
 /**
  * Relationship to an extent.
  * @enum {number}