gdb_server: remove target_handle_event from gdb event callback

In target_call_event_callbacks(), it will execute

1. target_handle_event (use Jim_EvalObj() to evaluate event
   statements in config files)
2. call user registered callbacks

Before calling user registered callbacks, target_handle_event has
been executed.  So, there is no need to call target_handle_event()
in gdb event callback.  It will execute event statements in config
files twice.

Change-Id: I84629e324fa3eb909907badf2319b4138ba89f07
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1372
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>