driver: char: rpivid: Destroy the legacy device on remove

The legacy name support created a new device that was never destroyed.
If the driver was unloaded and reloaded, it failed due to the
device already existing.

Fixes: "75f1d14c driver: char: rpivid - also support legacy name"
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>