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.

cumulus/parachains/runtimes/collectives/collectives-westend/Cargo.toml

@@ -77,7 +77,6 @@ pallet-collator-selection = { path = "../../../../pallets/collator-selection", d
 pallet-collective-content = { path = "../../../pallets/collective-content", default-features = false }
 parachain-info = { package = "staging-parachain-info", path = "../../../pallets/parachain-info", default-features = false }
 parachains-common = { path = "../../../common", default-features = false }
-testnets-common = { path = "../../../testnets-common", default-features = false }
 
 [build-dependencies]
 substrate-wasm-builder = { path = "../../../../../substrate/utils/wasm-builder", optional = true }
@@ -118,7 +117,6 @@ runtime-benchmarks = [
 	"polkadot-parachain-primitives/runtime-benchmarks",
 	"polkadot-runtime-common/runtime-benchmarks",
 	"sp-runtime/runtime-benchmarks",
-	"testnets-common/runtime-benchmarks",
 	"xcm-builder/runtime-benchmarks",
 	"xcm-executor/runtime-benchmarks",
 ]
@@ -216,7 +214,6 @@ std = [
 	"sp-transaction-pool/std",
 	"sp-version/std",
 	"substrate-wasm-builder",
-	"testnets-common/std",
 	"westend-runtime-constants/std",
 	"xcm-builder/std",
 	"xcm-executor/std",