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.

Cargo.lock

@@ -2668,7 +2668,6 @@ dependencies = [
  "staging-xcm-builder",
  "staging-xcm-executor",
  "substrate-wasm-builder",
- "testnets-common",
  "westend-runtime-constants",
 ]
 
@@ -19182,19 +19181,6 @@ dependencies = [
  "sp-weights",
 ]
 
-[[package]]
-name = "testnets-common"
-version = "1.0.0"
-dependencies = [
- "frame-support",
- "polkadot-core-primitives",
- "rococo-runtime-constants",
- "smallvec",
- "sp-runtime",
- "substrate-wasm-builder",
- "westend-runtime-constants",
-]
-
 [[package]]
 name = "textwrap"
 version = "0.16.0"