armv7m: Remove unused armv7m_nvic_acknowledge_irq() return value

armv7m: Remove unused armv7m_nvic_acknowledge_irq() return value

Having armv7m_nvic_acknowledge_irq() return the new value of
env->v7m.exception and its one caller assign the return value
back to env->v7m.exception is pointless. Just make the return
type void instead.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>