[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has since been superseded by new splicing options brought by recent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.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index c9bd0a9..19d1b04 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -101,7 +101,7 @@
 #define PR_O_TPXY_CLI	0x06000000	/* bind to the client's IP+port when connect()ing */
 #define PR_O_TPXY_MASK	0x06000000	/* bind to a non-local address when connect()ing */
 
-#define PR_O_TCPSPLICE	0x08000000      /* delegate data transfer to linux kernel's tcp_splice */
+/* unused : tcpsplice   0x08000000 */
 #define PR_O_CONTSTATS	0x10000000	/* continous counters */
 #define PR_O_HTTP_PROXY 0x20000000	/* Enable session to use HTTP proxy operations */
 #define PR_O_DISABLE404 0x40000000      /* Disable a server on a 404 response to a health-check */