net: dismiss sk_forward_alloc_get()

After the previous patch we can remove the forward_alloc_get
proto callback, basically reverting commit 292e6077 ("net: introduce
sk_forward_alloc_get()") and commit 66d58f04 ("net: use
sk_forward_alloc_get() in sk_get_meminfo()").

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250218-net-next-mptcp-rx-path-refactor-v1-5-4a47d90d7998@kernel.org


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