Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
* Move away from DT_LABEL.
* Move to DEVICE_DT_GET for non-behavior device access.
* Move various drivers to `gpio_spec_dt` and `DT` related macros.
* Remove mcp23017 while at it, since better upstream driver is
available.