Remove signal translations for unhandled signals
Remove signal translations for unhandled signals

The `from_signal_code` function is only called in the context of
dealing with signals that have been caught by our signal handler
function. However, several signals that we have conversions for are
not actually handled, so there's no point to having the conversions.

These were:
- `SIGILL`
- `SIGABRT`
- `SIGFPE`
- `SIGKILL` (that one *can't* be caught, anyway)
- `SIGSEGV`

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat