nixos: Remove modprobe.d/nixos.conf from initrd.

For example in VM tests, this causes firmware to be included in the
initrd. So until we have a better fix for adding early-stage module
options, I'll remove this.

Fixes a regression introduced by 0aa2c1dc

 and closes #3764.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>