Delete Kiln and Ropsten configs (#4038)

## Proposed Changes

Remove built-in support for Ropsten and Kiln via the `--network` flag. Both testnets are long dead and deprecated.

This shaves about 30MiB off the binary size, from 135MiB to 103MiB (maxperf), or 165MiB to 135MiB (release).

Author: michaelsproul

book/src/database-migrations.md

@@ -116,7 +116,7 @@ Several conditions need to be met in order to run `lighthouse db`:
 2. The command must run as the user that owns the beacon node database. If you are using systemd then
    your beacon node might run as a user called `lighthousebeacon`.
 3. The `--datadir` flag must be set to the location of the Lighthouse data directory.
-4. The `--network` flag must be set to the correct network, e.g. `mainnet`, `prater` or `ropsten`.
+4. The `--network` flag must be set to the correct network, e.g. `mainnet`, `prater` or `sepolia`.
 
 The general form for a `lighthouse db` command is:
 

common/eth2_config/src/lib.rs

@@ -278,26 +278,6 @@ define_hardcoded_nets!(
         // directory.
         GENESIS_STATE_IS_KNOWN
     ),
-    (
-        // Network name (must be unique among all networks).
-        kiln,
-        // The name of the directory in the `eth2_network_config/built_in_network_configs`
-        // directory where the configuration files are located for this network.
-        "kiln",
-        // Set to `true` if the genesis state can be found in the `built_in_network_configs`
-        // directory.
-        GENESIS_STATE_IS_KNOWN
-    ),
-    (
-        // Network name (must be unique among all networks).
-        ropsten,
-        // The name of the directory in the `eth2_network_config/built_in_network_configs`
-        // directory where the configuration files are located for this network.
-        "ropsten",
-        // Set to `true` if the genesis state can be found in the `built_in_network_configs`
-        // directory.
-        GENESIS_STATE_IS_KNOWN
-    ),
     (
         // Network name (must be unique among all networks).
         sepolia,