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

This reverts commit efec287c 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: I8f64380576053dce28f0f73ef06ccf0a27469ed1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>