Remove `testnets-common` (#2620)

`testnets-common` was introduced recently to start to separate testnet
parachain configurations from those of Polkadot and Kusama.

The `locks-review` and `polkadot-review` requirements are removed from
`parachains-common` in
https://github.com/paritytech/polkadot-sdk/pull/2564 and there are
[plans](https://github.com/paritytech/polkadot-sdk/pull/2564#discussion_r1410882115)
to move the Polkadot and Kusama contents of that package to the
fellowship, `testnets-common` is no longer needed.

This PR removes the crate and replaces uses of it in
`collectives-westend`, the only place it is currently used.