Revert "gtp: Destroy device along with udp socket's netns dismantle."

This reverts commit c986380c which is
commit eb28fd76 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Icb16f395345b5eb6bd40cb8f50780af8c8ffcf09
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>