inet6: Remove inet6_destroy_sock().

The last user of inet6_destroy_sock() is its wrapper inet6_cleanup_sock().
Let's rename inet6_destroy_sock() to inet6_cleanup_sock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>