Remove outdated ChainId::root references, and an unused file. (#4644)

## Motivation

Since the introduction of chain description blobs, `ChainId::root`
doesn't exist anymore, but there are still a few references to it.

Also, `applications_tests` isn't used anymore. It probably became
obsolete with the introduction of application blobs?

## Proposal

Fix the references and remove the unused file.

## Test Plan

CI

## Release Plan

- These changes _could_ be backported to `testnet_conway`.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: afck

CLI.md

@@ -1210,10 +1210,10 @@ Start a Local Linera Network
 * `--external-protocol <EXTERNAL_PROTOCOL>` — External protocol used, either `grpc` or `grpcs`
 
   Default value: `grpc`
-* `--with-faucet` — If present, a faucet is started using the chain provided by --faucet-chain, or `ChainId::root(1)` if not provided, as root 0 is usually the admin chain
+* `--with-faucet` — If present, a faucet is started using the chain provided by --faucet-chain, or the first non-admin chain if not provided
 
   Default value: `false`
-* `--faucet-chain <FAUCET_CHAIN>` — When using --with-faucet, this specifies the chain on which the faucet will be started. The chain is specified by its root number (0 for the admin chain, 1 for the first non-admin initial chain, etc)
+* `--faucet-chain <FAUCET_CHAIN>` — When using --with-faucet, this specifies the chain on which the faucet will be started. If this is `n`, the `n`-th non-admin chain (lexicographically) in the wallet is selected
 * `--faucet-port <FAUCET_PORT>` — The port on which to run the faucet server
 
   Default value: `8080`

linera-core/src/unit_tests/worker_tests.rs

@@ -1725,7 +1725,7 @@ where
         .inboxes
         .try_load_entry(&chain_3)
         .await?
-        .expect("Missing inbox for `chain_3` in `ChainId::root(1)`");
+        .expect("Missing inbox for `chain_3` in `chain_1`");
     assert_eq!(BlockHeight::ZERO, inbox.next_block_height_to_receive()?);
     assert_eq!(inbox.added_bundles.count(), 0);
     assert_matches!(
@@ -1993,7 +1993,7 @@ where
         .inboxes
         .try_load_entry(&chain_1)
         .await?
-        .expect("Missing inbox for `ChainId::root(1)` in `chain_2`");
+        .expect("Missing inbox for `chain_1` in `chain_2`");
     assert_eq!(BlockHeight::from(1), inbox.next_block_height_to_receive()?);
     assert_matches!(
         inbox

linera-execution/src/unit_tests/applications_tests.rs

@@ -1128,13 +1128,12 @@ pub enum NetCommand {
         external_protocol: String,
 
         /// If present, a faucet is started using the chain provided by --faucet-chain, or
-        /// `ChainId::root(1)` if not provided, as root 0 is usually the admin chain.
+        /// the first non-admin chain if not provided.
         #[arg(long, default_value = "false")]
         with_faucet: bool,
 
         /// When using --with-faucet, this specifies the chain on which the faucet will be started.
-        /// The chain is specified by its root number (0 for the admin chain, 1 for the first
-        /// non-admin initial chain, etc).
+        /// If this is `n`, the `n`-th non-admin chain (lexicographically) in the wallet is selected.
         #[arg(long)]
         faucet_chain: Option<u32>,