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).

testing/web3signer_tests/src/lib.rs

@@ -660,17 +660,17 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn ropsten_base_types() {
-        test_base_types("ropsten", 4250).await
+    async fn sepolia_base_types() {
+        test_base_types("sepolia", 4250).await
     }
 
     #[tokio::test]
-    async fn ropsten_altair_types() {
-        test_altair_types("ropsten", 4251).await
+    async fn sepolia_altair_types() {
+        test_altair_types("sepolia", 4251).await
     }
 
     #[tokio::test]
-    async fn ropsten_merge_types() {
-        test_merge_types("ropsten", 4252).await
+    async fn sepolia_merge_types() {
+        test_merge_types("sepolia", 4252).await
     }
 }