Remove the power regulator from the design.

Since this board is primarily for programming, it's fine to just get
power from the ICSP or FTDI headers in most cases, which should already
be regulated to 5V.

There was nowhere in the design for the power barrel jack to go anyway,
so this saves a little more space and also allows the ICSP header to move
closer to the SPI pins and thus fix some of the more awkward routes
from the previous iteration.