Remove trivial .fw file
Remove trivial .fw file

This brings me great sadness to lose, but it has become too painful to
maintain. It requires relx and rebar support in nerves_system_br. Since
neither are used (rebar is supported outside of Buildroot w/ mix) for
Nerves projects any longer, it is getting hard to justify time debugging
them when Nerves system builds break. I will find an alternative way of
debugging trivial images.

nerves-project/nerves_system_rpi0,nerves-project/nerves_system_rpi0