Delete RuntimeVariableList::from_vec (#7930)

This method is a footgun because it truncates the list. It is the source of a recent bug:

- https://github.com/sigp/lighthouse/pull/7927


  - Delete uses of `RuntimeVariableList::from_vec` and replace them with `::new` which does validation and can fail.
- Propagate errors where possible, unwrap in tests and use `expect` for obviously-safe uses (in `chain_spec.rs`).

Cargo.lock

@@ -8978,6 +8978,7 @@ dependencies = [
  "safe_arith",
  "serde",
  "smallvec",
+ "ssz_types",
  "state_processing",
  "strum",
  "superstruct",