net: drop !NET_MULTI code

This is long over due.  All but two net drivers have been converted, but
those have now been dropped.

The only thing left to do is actually delete all references to NET_MULTI
and code that is compiled when that is not defined.  So here we scrub the
core code.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

api/api_net.c

@@ -34,10 +34,6 @@ DECLARE_GLOBAL_DATA_PTR;
 #define DEBUG
 #undef DEBUG
 
-#if !defined(CONFIG_NET_MULTI)
-#error "API/net is currently only available for platforms with CONFIG_NET_MULTI"
-#endif
-
 #ifdef DEBUG
 #define debugf(fmt, args...) do { printf("%s(): ", __func__); printf(fmt, ##args); } while (0)
 #else