Disable old global lock system

Remove the old global locking system. We lock only the accounts that we
currently sync, so you can invoke OfflineImap multiple times now as long
as you sync different accounts. This system is compatible with all
releases >= 6.4.0, so don't run older releases simultanous to this one.

This mostly reverts commit 0d95651417,
disabling the old global lock system that we had in parallel to the new one.

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

offlineimap/accounts.py

@@ -191,7 +191,6 @@ class SyncableAccount(Account):
 
     def lock(self):
         """Lock the account, throwing an exception if it is locked already"""
-        # Take a new-style per-account lock
         self._lockfd = open(self._lockfilepath, 'w')
         try:
             fcntl.lockf(self._lockfd, fcntl.LOCK_EX|fcntl.LOCK_NB)