fix(PathRequest): remove incorrect assert (XRPLF#4743)

The assert is saying that the only reason `pathFinder` would be null is
if the request was aborted (connection dropped, etc.). That's what
`continueCallback()` checks. But that is very clearly not true if you
look at `getPathFinder`, which calls `findPaths`, which can return false
for many reasons.

Fix XRPLF#4744

src/ripple/app/paths/PathRequest.cpp

@@ -538,7 +538,6 @@ PathRequest::findPaths(
  continueCallback);
  if (!pathfinder)
  {
- assert(continueCallback && !continueCallback());
  JLOG(m_journal.debug()) << iIdentifier << " No paths found";
  continue;
  }