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>
diff --git a/README b/README
index dfc7ada..0868531 100644
--- a/README
+++ b/README
@@ -3511,12 +3511,10 @@
 
   bootstopkey	- see CONFIG_AUTOBOOT_STOP_STR
 
-  ethprime	- When CONFIG_NET_MULTI is enabled controls which
-		  interface is used first.
+  ethprime	- controls which interface is used first.
 
-  ethact	- When CONFIG_NET_MULTI is enabled controls which
-		  interface is currently active. For example you
-		  can do the following
+  ethact	- controls which interface is currently active.
+		  For example you can do the following
 
 		  => setenv ethact FEC
 		  => ping 192.168.0.1 # traffic sent on FEC