hw/misc/stm32f2xx_syscfg: Remove extraneous IRQ

The system configuration controller (SYSCFG) doesn't have
any output IRQ (and the INTC input #71 belongs to the UART6).
Remove the invalid code.

Fixes: db635521 ("stm32f205: Add the stm32f205 SoC")
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201107193403.436146-3-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>