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>