Remove nSproutValue TODO from CDiskBlockIndex

Block indices are flushed to disk when they are marked as dirty, and this
happens via enough distinct pathways that it would be sufficiently complex to
update nSproutValue via all of them. Thus it is necessary to be able to
serialize "no value" for writes by upgraded clients.

src/chain.h

@@ -355,8 +355,6 @@ public:
 
         // Only read/write nSproutValue if the client version used to create
         // this index was storing them.
-        // TODO: See if we can get away with not serializing the boost::optional
-        // one-byte header, without requiring users to reindex on upgrade.
         if ((nType & SER_DISK) && (nVersion >= SPROUT_VALUE_VERSION)) {
             READWRITE(nSproutValue);
         }