routing/router_test: remove preimage overwrite

The test case's preimage was (mistakenly) overwritten after crafting the
lightning payment, causing the parts of the testcases use the same
preimage causing problems when we are using the payment hash and
preimage in the mock control tower to distinguish paymennts.

routing/router_test.go

@@ -3025,8 +3025,6 @@ func TestRouterPaymentStateMachine(t *testing.T) {
 			PaymentHash: payHash,
 		}
 
-		copy(preImage[:], bytes.Repeat([]byte{9}, 32))
-
 		router.cfg.SessionSource = &mockPaymentSessionSource{
 			routes: test.routes,
 		}