nwfilter: remove recently added workaround define for macos x

This reverses commit 04c3704, which added a define to nwfilter to
allow libvirtd compilation on Mac OS X.  Stefan Bergers commit, 2e7294d,
is the proper solution, removing the requirement for nwfilter on non-Linux.

src/nwfilter/nwfilter_learnipaddr.c

@@ -38,11 +38,6 @@
 #include <net/if_arp.h>
 #include <intprops.h>
 
-// Add Linux define missing from OSX
-#ifndef ETH_ALEN
-# define ETH_ALEN       6               /* Octets in one ethernet addr   */
-#endif
-
 #include "internal.h"
 
 #include "buf.h"