msf2-soc, stellaris: Don't wire up SYSRESETREQ

msf2-soc, stellaris: Don't wire up SYSRESETREQ

The MSF2 SoC model and the Stellaris board code both wire
SYSRESETREQ up to a function that just invokes
    qemu_system_reset_request(SHUTDOWN_CAUSE_GUEST_RESET);
This is now the default action that the NVIC does if the line is
not connected, so we can delete the handling code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200728103744.6909-4-peter.maydell@linaro.org