lnonion/trampoline: stop double counting HMAC size

`OnionHopsDataSingle.to_bytes()` already accounts for the
onions hmac internally, it seems incorrect that we were
additionally adding `PER_HOP_HMAC_SIZE` to the payload size
of the payload returned by `.to_bytes()`.

Author: f321x

electrum/lnonion.py

@@ -228,7 +228,7 @@ def new_onion_packet(
     payload_size = 0
     for i in range(num_hops):
         # FIXME: serializing here and again below. cache bytes in OnionHopsDataSingle? _raw_bytes_payload?
-        payload_size += PER_HOP_HMAC_SIZE + len(hops_data[i].to_bytes())
+        payload_size += len(hops_data[i].to_bytes())
     if trampoline:
         data_size = payload_size
     elif onion_message:

electrum/trampoline.py

@@ -10,7 +10,7 @@
 
 from .lnutil import LnFeatures, PaymentFeeBudget, FeeBudgetExceeded
 from .lnonion import (
-    calc_hops_data_for_payment, new_onion_packet, OnionPacket, PER_HOP_HMAC_SIZE
+    calc_hops_data_for_payment, new_onion_packet, OnionPacket
 )
 from .lnrouter import TrampolineEdge, is_route_within_budget, LNPaymentTRoute
 from .lnutil import NoPathFound
@@ -418,8 +418,7 @@ def create_trampoline_onion(
         payload = dict(hops_data[index].payload)
         # try different r_tag order on each attempt
         invoice_routing_info = random_shuffled_copy(route[index].invoice_routing_info)
-        remaining_payload_space = TRAMPOLINE_HOPS_MAX_DATA_SIZE \
-                                  - sum(len(hop.to_bytes()) + PER_HOP_HMAC_SIZE for hop in hops_data)
+        remaining_payload_space = TRAMPOLINE_HOPS_MAX_DATA_SIZE - sum(len(hop.to_bytes()) for hop in hops_data)
         routing_info_to_use = []
         for encoded_r_tag in invoice_routing_info:
             if remaining_payload_space < 50: