gdb_server: only trigger once the event gdb-detach at gdb quit

gdb_server: only trigger once the event gdb-detach at gdb quit

When GDB quits (e.g. with "quit" command) we first execute
gdb_detach() to reply "OK" then, at GDB disconnect (either TCP
or pipe connection type), we execute gdb_connection_closed().
In case GDB is killed or it crashes, OpenOCD only executes the
latter when detects the disconnection.
Both gdb_detach() and gdb_connection_closed() trigger the event
TARGET_EVENT_GDB_DETACH thus getting it triggered twice on clean
GDB quit.

Do not trigger the event TARGET_EVENT_GDB_DETACH in gdb_detach()
and let only gdb_connection_closed() to handle it.

Change-Id: Iacf035c855b8b3e2239c1c0e259c279688b418ee
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4585
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>