Revert "build: fix configure detection of if_bridge.h on RHEL 6"

This reverts commit 2df8d991.

The change breaks configure on any recent Fedora platform
(cherry picked from commit 43cee32f)

configure.ac

@@ -997,14 +997,7 @@ if test "$with_linux" = "yes"; then
   if test "$with_qemu" = "yes" || test "$with_lxc" = "yes" ; then
     AC_CHECK_HEADERS([linux/param.h linux/sockios.h linux/if_bridge.h linux/if_tun.h],,
       [AC_MSG_ERROR([You must install kernel-headers in order to compile libvirt with QEMU or LXC support])],
-      [[/* The kernel folks broke their headers when used with particular
-         * glibc versions; although the structs are ABI compatible, the
-         * C type system doesn't like struct redefinitions.  We work around
-         * the problem here in the same manner as in virnetdevbridge.c.  */
-        #include <netinet/in.h>
-        #define in6_addr in6_addr_
-        #define sockaddr_in6 sockaddr_in6_
-        #define ipv6_mreq ipv6_mreq_
+      [[#include <netinet/in.h>
       ]])
   fi
 fi