fix(cds-plugin-ui5): remove activation hook since it's not needed (#900)

Instead of supporting both variants, activate hook in object vs.
Promise, the activate hook has been completely removed to avoid
a switch statement based on version. This ensures simplicity of
code and it wasn't needed at all...

Fixes #899

Author: petermuessig

Diff for: packages/cds-plugin-ui5/cds-plugin.js

@@ -160,12 +160,3 @@ if (!skip) {
 		bootstrapped();
 	});
 }
-
-// return callback for plugin activation
-module.exports = {
-	activate: function activate(conf) {
-		if (!skip) {
-			log.debug("activate", conf);
-		}
-	},
-};