bigfile/zodb: Comments for ZBigFile + LivePersistent are ok

Again, leftover from 4174b84a (bigfile: BigFile backend to store data in
ZODB).

bigfile/file_zodb.py

@@ -144,7 +144,6 @@ class ZBlk(Persistent):
         Persistent._p_invalidate(self)
 
 
-# XXX merge Persistent/BigFile comments into 1 place
 # helper for ZBigFile - just redirect loadblk/storeblk back
 # (because it is not possible to inherit from both Persistent and BigFile at
 #  the same time - see below)