Remove obsolete comment about get()/remove() concurrency.

This paragraph has most likely become obsolete with commit d8c3fff00b93.
At the moment, the interaction of get() and remove() is exactly as one
would expect: if get() gets the lock before remove(), the block is
removed - otherwise it remains. The comment muddles the water more
than clarifying the behavior.

src/s3ql/block_cache.py

@@ -724,10 +724,6 @@ def get(self, inode, blockno):
         """Get file handle for block `blockno` of `inode`
 
         This method releases the global lock.
-
-        Note: if `get` and `remove` are called concurrently, then it is
-        possible that a block that has been requested with `get` and
-        passed to `remove` for deletion will not be deleted.
         """
 
         #log.debug('started with %d, %d', inode, blockno)