nixos/bitcoind: remove PermissionsStartOnly

Unneded because there are no extra commands like ExecStartPre

nixos/modules/services/networking/bitcoind.nix

···
         NoNewPrivileges = "true";
         PrivateDevices = "true";
         MemoryDenyWriteExecute = "true";
-
-        # Permission for preStart
-        PermissionsStartOnly = "true";
       };
     };
     users.users.${cfg.user} = {