routing: map insufficient local bandwidth error to no path

With mpp it isn't possible anymore for findPath to determine that there
isn't enough local bandwidth. The full payment amount isn't known at
that point.

In a follow-up, this payment outcome can be reintroduced on a higher
level (payment lifecycle).

routing/pathfind_test.go

@@ -2809,37 +2809,6 @@ func TestRouteToSelf(t *testing.T) {
 	ctx.assertPath(path, []uint64{1, 3, 2})
 }
 
-// TestInsufficientBalance tests that a dedicated error is returned for
-// insufficient local balance.
-func TestInsufficientBalance(t *testing.T) {
-	t.Parallel()
-
-	testChannels := []*testChannel{
-		symmetricTestChannel("source", "target", 100000, &testChannelPolicy{
-			Expiry:      144,
-			FeeBaseMsat: 500,
-		}, 1),
-	}
-
-	ctx := newPathFindingTestContext(t, testChannels, "source")
-	defer ctx.cleanup()
-
-	paymentAmt := lnwire.NewMSatFromSatoshis(100)
-	target := ctx.keyFromAlias("target")
-
-	ctx.graphParams.bandwidthHints = map[uint64]lnwire.MilliSatoshi{
-		1: lnwire.NewMSatFromSatoshis(50),
-	}
-
-	// Find the best path to self. We expect this to be source->a->source,
-	// because a charges the lowest forwarding fee.
-	_, err := ctx.findPath(target, paymentAmt)
-	if err != errInsufficientBalance {
-		t.Fatalf("expected insufficient balance error, but got: %v",
-			err)
-	}
-}
-
 type pathFindingTestContext struct {
 	t                 *testing.T
 	graphParams       graphParams