rv/rvgen: remove unused sys import from dot2c

rv/rvgen: remove unused sys import from dot2c

The sys module was imported in the dot2c frontend script but never
used. This import was likely left over from earlier development or
copied from a template that required sys for exit handling.

Remove the unused import to clean up the code and satisfy linters
that flag unused imports as errors.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20260223162407.147003-13-wander@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>