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

This reverts commit 036f8d81 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: Iba39eb18a9bbb97d865f1a93dc28807e4191ee0e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>