l2tp: Remove duplicate included header file trace.h

Remove duplicate included header file trace.h and the following warning
reported by make includecheck:

  trace.h is included more than once

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Link: https://patch.msgid.link/20240703061147.691973-2-thorsten.blum@toblux.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/l2tp/l2tp_core.c

@@ -60,7 +60,6 @@
 #include <linux/atomic.h>
 
 #include "l2tp_core.h"
-#include "trace.h"
 
 #define CREATE_TRACE_POINTS
 #include "trace.h"