tailor: make bitcoin modules truly Sovran-only (lnd-only) and delete stubs.nix

- lndconnect.nix: rewrite to lnd-only (320 lines removed) - remove
  services.clightning.plugins.clnrest.lnconnect and
  services.clightning-rest.lndconnect (Sovran only uses services.lnd.lndconnect for Zeus)
  Fixes: 'services.clightning.plugins.clnrest.address does not exist' on f13ff45
  and 'attribute enable missing' at lndconnect.nix:216

- btcpayserver.nix: remove liquidd (lbtc) dead branch - Sovran uses lbtc=false
  * remove inherit (config.services) bitcoind liquidd -> just bitcoind
  * lbtc chains/rpc handling is dead code, keep guards but no need for liquidd service

- mempool.nix: remove fulcrum branch - Sovran uses electrs only
- rtl.nix: remove lightning-loop block and clightning service block (lnd only)
- Delete modules/bitcoin/stubs.nix entirely - no dead service references left
  * liquidd, fulcrum, lightning-loop, joinmarket now not referenced at all
  * clightning already handled via guards, not stubs
- Result: modules/bitcoin/ is now 100% tailored to Sovran (bitcoind+knots, lnd, electrs, rtl, btcpayserver, mempool)
  No more stubs whack-a-mole on any nixos-unstable

modules/bitcoin/mempool.nix

@@ -170,8 +170,7 @@ let
 
   inherit (config.services)
     bitcoind
-    electrs
-    fulcrum;
+    electrs;
 
   torSocket = config.services.tor.client.socksListenAddress;
 
@@ -241,8 +240,7 @@ in {
   config = mkIf cfg.enable {
     services.bitcoind.txindex = true;
     services.electrs.enable = mkIf (cfg.electrumServer == "electrs" ) true;
-    # vendored fix: fulcrum may not exist
-    services.fulcrum.enable = mkIf (cfg.electrumServer == "fulcrum" && config.services ? fulcrum) true;
+    # vendored fix: fulcrum may not exist    # fulcrum removed - Sovran uses electrs only
     services.mysql = {
       enable = true;
       package = pkgs.mariadb;