ipaccess-proxy: remove leftover option `--udp'

It seems it remains unimplemented, remove it.
diff --git a/openbsc/src/ipaccess/ipaccess-proxy.c b/openbsc/src/ipaccess/ipaccess-proxy.c
index dd0a739..4dd6ee5 100644
--- a/openbsc/src/ipaccess/ipaccess-proxy.c
+++ b/openbsc/src/ipaccess/ipaccess-proxy.c
@@ -1145,7 +1145,6 @@
 			{"log-level", 1, 0, 'e'},
 			{"listen", 1, 0, 'l'},
 			{"bsc", 1, 0, 'b'},
-			{"udp", 1, 0, 'u'},
 			{0, 0, 0, 0}
 		};