Removes the object corresponding to key from the cache.
If the object was removed successfully, the cache calls contp back with the
TS_EVENT_CACHE_REMOVE. If the object was not found in the cache,
the cache calls contp back with the event
In both of these callbacks, the user (contp) does not have to do
anything. The user does not get any vconnection from the cache, since
no data needs to be transferred. When the cache calls contp back with
TS_EVENT_CACHE_REMOVE, the remove has already been committed.