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

This reverts commit 5f167834 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: I3cd0c10a77026947c74b214fd31e6d4bda525c65
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>