Remove keep-unfeasible documentation.

The code was removed in 0111f5c1.

babeld.c

@@ -873,7 +873,7 @@ main(int argc, char **argv)
             "               "
             "[-g port] [-G port] [-k metric] [-A metric] [-s] [-l] [-w] [-r]\n"
             "               "
-            "[-u] [-t table] [-T table] [-c file] [-C statement]\n"
+            "[-t table] [-T table] [-c file] [-C statement]\n"
             "               "
             "[-d level] [-D] [-L logfile] [-I pidfile]\n"
             "               "

babeld.man

@@ -96,11 +96,6 @@ derived from the MAC address of the first interface, which is easier
 to debug and more reliably prevents routing loops but may sometimes
 cause a node to be unreachable for 120 seconds just after boot.
 .TP
-.B \-u
-Do not flush unfeasible (useless) routes.  This is useful in order to
-announce more information to a front-end (see
-.BR \-g ).
-.TP
 .BI \-d " level"
 Debug level.  A value of 1 requests a routing table dump at every
 iteration through the daemon's main loop.  A value of 2 additionally