Remove comment about passing allocator to KeyCache:

After some discussion on https://github.com/ripple/rippled/pull/2595
we have decided that the allocator should not be plumbed through
the KeyCache class template. As such, remove the comment suggesting
to push the allocator through.

src/ripple/basics/KeyCache.h

@@ -34,7 +34,6 @@ namespace ripple {
     older than the maximum age they are eligible for removal during a
     call to @ref sweep.
 */
-// VFALCO TODO Figure out how to pass through the allocator
 template <
     class Key,
     class Hash = hardened_hash <>,