Revert "tun: call dev_get_valid_name() before register_netdevice()"

This reverts commit 0ad646c8.

As noticed by Jakub, this is no longer needed after
commit 11fc7d5a ("tun: fix memory leak in error path")

This no longer exports dev_get_valid_name() for the exclusive
use of tun driver.

Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>