selftests/net: Fix ./ns-XXXXXX not cleanup

```
readonly STATS="$(mktemp -p /tmp ns-XXXXXX)"
readonly BASE=`basename $STATS`
```
It could be a mistake to write to $BASE rather than $STATS, where $STATS
is used to save the NSTAT_HISTORY and it will be cleaned up before exit.

Although since we've been creating the wrong file this whole time and
everything worked, it's fine to remove these 2 lines completely

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Link: https://patch.msgid.link/20241030005943.400225-1-lizhijian@fujitsu.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>