Remove redundant `serializationHelper` functions

[DOBEN]

Task description

Smart contract developers do not need to know the serializationHelperParameter (CIS3 standard) in their dApps anymore after the merge of:
Concordium/concordium-dapp-examples#83. To reduce the size of the smart contract code (smaller wasm modules save on deployment and executing costs), the not needed serializationHelper function should be removed.

Sub-tasks

• Remove function in cis2-multi
  https://github.com/Concordium/concordium-rust-smart-contracts/blob/main/examples/cis2-multi/src/lib.rs#L1228
• Remove function in cis3-nft-sponsored-txs
  https://github.com/Concordium/concordium-rust-smart-contracts/blob/main/examples/cis3-nft-sponsored-txs/src/lib.rs#L710
• The parameter is used as a magicNumber without comment in the browser wallet. Document the parameter and how to re-create it in the browser wallet.
  https://github.com/Concordium/concordium-browser-wallet/blob/f30327a57797d69801be7def2893161acbd341b0/packages/browser-wallet/src/popup/pages/SignCIS3Message/SignCIS3Message.tsx#L29