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

This reverts commit bb11f992 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: I2bffc92bf02373ca3e34c4c818bc0515877eb5ae
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>