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.
* Previous version of multiple overrides of the default value of ZMK_USB
were problematic. Move to using board _defconfig files for the
defaults for those, along with proper `depends on` for ZMK_USB that
accounts for split and split roles.
* Remove MISO pin from pinctrl for BDN9 rev2 board, to avoid
the 4th switch being set up for an alternate function, which
was causing spurious key press events to be triggered.