staging: zcache: clean TODO list

Cleanup TODO list since support zero-filled pages more efficiently has
already done by this patchset.

Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/TODO

@@ -61,5 +61,4 @@ ZCACHE FUTURE NEW FUNCTIONALITY
 
 A. Support zsmalloc as an alternative high-density allocator
     (See https://lkml.org/lkml/2013/1/23/511)
-B. Support zero-filled pages more efficiently
-C. Possibly support three zbuds per pageframe when space allows
+B. Possibly support three zbuds per pageframe when space allows