Correctly unhook from delete_post when importing

Fixes #121. This bug was actually mostly cosmetic, as it looks like
the post is getting imported but the semaphore was
short-circuiting the import process because I incorrectly
unhooked the controller from the `delete_post` hook.

lib/controller.php

@@ -62,7 +62,7 @@ public function pull_posts() {
 
 		$this->app->semaphore()->lock();
 		remove_action( 'save_post', array( $this, 'export_post' ) );
-		remove_action( 'save_post', array( $this, 'delete_post' ) );
+		remove_action( 'delete_post', array( $this, 'delete_post' ) );
 
 		$result = $this->app->import()->payload( $payload );