mptcp: remove pr_fallback()

We can now track fully the fallback status of a given connection via the
relevant mibs, the mentioned helper is redundant. Remove it completely.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250723-net-next-mptcp-track-fallbacks-v1-2-a83cce08f2d5@kernel.org


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