Replace plated-through mounting holes with unplated vias

Replace plated-through mounting holes with unplated vias

The oshpark boards I got didn't consistently fit over the mounting
pegs in the boxes. I'm hoping that the unplated holes will be more
consistent. And, if not, at least this board won't count on the ground
conduction through these holes and so I can get a 130mil drill and
clean them out.

Signed-off-by: Keith Packard <keithp@keithp.com>