REGTESTS: remove unneeded experimental-mode in cli add server test

The experimental-mode cli command is only required for the moment for
'add server'. Remove it for the 'enable server' command.
diff --git a/reg-tests/server/cli_add_server.vtc b/reg-tests/server/cli_add_server.vtc
index 67e7252..a6dcc8d 100644
--- a/reg-tests/server/cli_add_server.vtc
+++ b/reg-tests/server/cli_add_server.vtc
@@ -81,7 +81,7 @@
 } -run
 
 haproxy h1 -cli {
-	send "experimental-mode on; enable server test/s1"
+	send "enable server test/s1"
 }
 
 client c3 -connect ${h1_feS_sock} {