Remove non-standard increase_allowance and decrease_allowance from ERC20

[milancermak]

🧐 Motivation

The Starknet equivalent to the recent ERC20 update of Solidity contracts. Having these methods is even less necessary on Starknet thanks to AA and multicall.

Additional benefit of removing them is slightly reduced contract size.

Since the Cairo 2 contracts haven't been officially released, I think it's a good opportunity to squeeze this in 🤞

[martriay]

+1, although I don't think this will make it to the release.

[TAdev0]

@martriay I just saw your message on the starknet Cairo core stars channel. May I work on this issue?
Thank you.

[martriay]

Thanks for volunteering! You can tackle either this or #841, but I suggest you start with one at the time :)

[TAdev0]

sure! will work on this one tonight :)