Remove two SPARKFUN_SAMD51_THINGS_PLUS pin definitions. There were
definitions of TXD and RXD, but these pins do not exist on the board. They
were only shown in the schematics.
Also remove any reference to LED_. This is just a text change, no
functional change.
Changes are:
- Remove the LED_Pxxx definitions from pins.csv, now that the LED class is
gone.
- Remove the '-' lines.
- Add default lines for USB and SWCLK, SWDIO.