Don't setDaemon explicitly, it's done inherently

All ExitNotifyThreads and InstanceLimitThreads are setDaemon(True) in their
constructor, so there is no need to do that again in the code.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/Base.py

@@ -341,7 +341,6 @@ class BaseFolder(object):
                     target = self.copymessageto,
                     name = "Copy message from %s:%s" % (self.repository, self),
                     args = (uid, dstfolder, statusfolder))
-                thread.setDaemon(1)
                 thread.start()
                 threads.append(thread)
             else: