avr32: Get rid of the .flashprog section

The .flashprog section was only needed back when we were running
directly from flash, and it's even more useless on NGW100 since it
uses the CFI flash driver which never used this workaround in the
first place.

Remove it on STK1000 as well, and get rid of all the associated code and
annotations.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>