avr32: Remove DMATEST from defconfigs

In the defconfigs for ATNGW100 and ATSTK100[236] the DMA Test driver is
compiled as a module.  This means systems built with *_defconfig +
CONFIG_MODULES=n are unusable as the 3 dma test channels monopolise the
CPU.

I 'spose Haavard uses this module a lot but IMO it isn't really
something needed on all eval boards by default.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>