(Like getting a mobile phone sort makes having a stationary phone with a landline redundant. Or a microwave oven can make a microwave and/or oven redundant. There is no use for it anymore because you got something new that does the job, usually beter or does the old job plus more)
Historically if a person was fired it was because of misconduct and the employer would take their tools from them and burn them - hence "fired". If they had not behaved badly their tools would be handed to them in a sack to take to another job hence the term "getting the sack"