Revert "gdbstub: Do not kill target in system emulation mode"

The requirements described in this patch are implemented by "Add GDB
qAttached support".

This reverts commit 00e94dbc.

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -888,11 +888,9 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
             goto unknown_command;
         }
     case 'k':
-#ifdef CONFIG_USER_ONLY
         /* Kill the target */
         fprintf(stderr, "\nQEMU: Terminated via GDBstub\n");
         exit(0);
-#endif
     case 'D':
         /* Detach packet */
         gdb_breakpoint_remove_all();