This reverts commit 220b1386ab.
because it causes crashes when saving sessions.
_channel_map[] must not be empty, otherwise
Program terminated with signal SIGSEGV, Segmentation fault.
snprintf (buf, sizeof (buf), "%d", _channel_map[chn]);
#0 ARDOUR::MIDITrigger::get_state() (this=0x55d676b4c600) at ../libs/ardour/triggerbox.cc:1962
#1 0x00007f58ccb40b12 in ARDOUR::TriggerBox::get_state() (this=0x55d676b4d720) at ../libs/ardour/triggerbox.cc:33