assethub: Use memorydb read/write weights

[alexggh]

Post changes and optimisation from paritytech/polkadot-sdk#6131, after stable2506 is integrated in the runtime assethub-kusama and assethub-polkadot should be modified to use the read/write weights measured in validation context.

Examples of PR that changed westend: paritytech/polkadot-sdk#8359.

• Asset-hub-kusama
• Asset-hub-polkadot

[bkontur]

@ggwpez based on your comment, is this sensitive change for AHM? Can be done anytime, before or after?

[bkchr]

We basically just use a different set of weights that should be closer to reality.