babeld: fix interface bug, simplify code.

Perhaps could it be able to free already free memory (so free(NULL)),
in function interface_reset().  On other hand, it initiated untracked
interfaces, raising (at least) inappropriate messages. Finally, I
remove the BABEL_IF_IS_ENABLE flag, witch was not really usefull.
Note the test if_up isn't weaker, because (...IS_UP => ...IS_ENABLE).
diff --git a/babeld/babel_interface.h b/babeld/babel_interface.h
index 5b551fb..ec69bff 100644
--- a/babeld/babel_interface.h
+++ b/babeld/babel_interface.h
@@ -99,7 +99,6 @@
 #define BABEL_IF_SPLIT_HORIZON (1 << 2)
 #define BABEL_IF_LQ            (1 << 3)
 #define BABEL_IF_FARAWAY       (1 << 4)
-#define BABEL_IF_IS_ENABLE     (1 << 7)
 
 /* Only INTERFERING can appear on the wire. */
 #define BABEL_IF_CHANNEL_UNKNOWN 0
@@ -111,9 +110,7 @@
 {
     return (if_is_operative(ifp) &&
             ifp->connected != NULL &&
-            babel_get_if_nfo(ifp) != NULL &&
-            (babel_get_if_nfo(ifp)->flags & BABEL_IF_IS_UP) &&
-            (babel_get_if_nfo(ifp)->flags & BABEL_IF_IS_ENABLE));
+            (babel_get_if_nfo(ifp)->flags & BABEL_IF_IS_UP));
 }
 
 /* types: