1) clean up commented print statements

2) change an incorrect comment in caches.py

src/calibre/library/caches.py

@@ -49,7 +49,7 @@ class MetadataBackup(Thread): # {{{
     def run(self):
         while self.keep_running:
             try:
-                time.sleep(2) # Limit to two per second
+                time.sleep(2) # Limit to one book per two seconds
                 (id_, sequence) = self.db.get_a_dirtied_book()
                 if id_ is None:
                     continue

src/calibre/library/database2.py

@@ -827,7 +827,6 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             try:
                 book_ids = self.data.parse(query)
             except:
-                import traceback
                 traceback.print_exc()
                 return identical_book_ids
             for book_id in book_ids: