htlcswitch/hodl/config_production: hide cli hodl flags in prod

This commit replaces the debug Config struct with an empty
one, so that the command line flags are hidden in production
builds.

Production help before commit:

Tor:
      --tor.active
      --tor.socks=
      --tor.dns=
      --tor.streamisolation
      --tor.control=
      --tor.v2
      --tor.v2privatekeypath=
      --tor.v3

hodl:
      --hodl.exit-settle
      --hodl.add-incoming
      --hodl.settle-incoming
      --hodl.fail-incoming
      --hodl.add-outgoing
      --hodl.settle-outgoing
      --hodl.fail-outgoing
      --hodl.commit
      --hodl.bogus-settle

Help Options:
  -h, --help

Production help after commit:

Tor:
      --tor.active
      --tor.socks=
      --tor.dns=
      --tor.streamisolation
      --tor.control=
      --tor.v2
      --tor.v2privatekeypath=
      --tor.v3

Help Options:
  -h, --help

htlcswitch/hodl/config_production.go

@@ -0,0 +1,11 @@
+// +build !debug
+
+package hodl
+
+// Config is an empty struct disabling command line hodl flags in production.
+type Config struct{}
+
+// Mask in production always returns MaskNone.
+func (c *Config) Mask() Mask {
+	return MaskNone
+}