mgw: Drop unneeded mp_enable_conn_timeout_test

Latest osmo-mgw release is 1.7.0, so this param is not needed at all.
Furthermore, the config can be moved to .cfg.

Change-Id: I537c0f5fd6f9e18e111c773c0e42e5f1120ce2f4
diff --git a/mgw/osmo-mgw.cfg b/mgw/osmo-mgw.cfg
index b27a50c..edc8190 100644
--- a/mgw/osmo-mgw.cfg
+++ b/mgw/osmo-mgw.cfg
@@ -31,6 +31,7 @@
   sdp audio-payload name GSM
   sdp audio-payload send-ptime
   sdp audio-payload send-name
+  conn-timeout 0
   loop 0
   number endpoints 31
   allow-transcoding