tcp: remove sk_{tr}x_skb_cache

tcp: remove sk_{tr}x_skb_cache

This reverts the following patches :

- commit eb695b16d383 ("tcp: fix compile error if !CONFIG_SYSCTL")
- commit e9b62202b2b5 ("tcp: fix zerocopy and notsent_lowat issues")
- commit 7ddae09f6567 ("tcp: add one skb cache for tx")
- commit 8abd0fe37af9 ("tcp: add one skb cache for rx")

Having a cache of one skb (in each direction) per TCP socket is fragile,
since it can cause a significant increase of memory needs,
and not good enough for high speed flows anyway where more than one skb
is needed.

We want instead to add a generic infrastructure, with more flexible
per-cpu caches, for alien NUMA nodes.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>