selftests: mptcp: lib: stats: remove nstat rate columns

selftests: mptcp: lib: stats: remove nstat rate columns

With the MPTCP selftests, the nstat daemon is not used. It means that
the last column (the rate) is always 0.0, and that's not something
interesting to display.

Then, this last column can be filtered out.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251114-net-next-mptcp-sft-count-cache-stats-timeout-v1-3-863cb04e1b7b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>