Westend system chains: clean-up non permanent migrations we don't need anymore

[sigurpol]

As noted during the review of #11763, on Westend system chains we have many migrations not meant to be permanent , run long time ago that can be safely removed.

Examples:

• https://github.com/paritytech/polkadot-sdk/pull/11763/changes#r3086039522
• https://github.com/paritytech/polkadot-sdk/pull/11763/changes#r3086395866
• https://github.com/paritytech/polkadot-sdk/pull/11763/changes#r3086056263