Fix more problems introduced by 6adb2ec

- Renamed canSendToAccount to canWithdraw, because the semantics are
  a little different from a payment. Notably, if withdrawing to self,
  you can still include a destination tag.
- Simplified the interface to canWithdraw to just pass in the
  STTx.
- preflightDestinationAndTag is pretty pointless now, so removed it.

src/test/app/LoanBroker_test.cpp

@@ -384,13 +384,6 @@ class LoanBroker_test : public beast::unit_test::suite
                 destination(AccountID{}),
                 ter(temMALFORMED));
 
-            // If a destination tag is specified, a destination must be
-            // specified, too
-            env(coverWithdraw(alice, keylet.key, vault.asset(1)),
-                dtag(123),
-                ter(temMALFORMED));
-            verifyCoverAmount(10);
-
             // Withdraw some of the cover amount
             env(coverWithdraw(alice, keylet.key, vault.asset(7)));
             env.close();