[MEDIUM] remove old experimental tcpsplice option

[MEDIUM] remove old experimental tcpsplice option

This Linux-specific option was never really used in production and
has since been superseded by new splicing options brought by recent
Linux kernels.

It caused several particular cases in the code because the kernel
would take care of the session without haproxy being able to do
anything on it, which became hard to handle in the new architecture.

Let's simply get rid of it now that there is a replacement available.