all: remove Kiln testnet (#26522)

Kiln was deprecated after the merge.

cmd/geth/main.go

@@ -285,9 +285,6 @@ func prepare(ctx *cli.Context) {
 	case ctx.IsSet(utils.SepoliaFlag.Name):
 		log.Info("Starting Geth on Sepolia testnet...")
 
-	case ctx.IsSet(utils.KilnFlag.Name):
-		log.Info("Starting Geth on Kiln testnet...")
-
 	case ctx.IsSet(utils.DeveloperFlag.Name):
 		log.Info("Starting Geth in ephemeral dev mode...")
 		log.Warn(`You are running Geth in --dev mode. Please note the following:
@@ -316,7 +313,6 @@ func prepare(ctx *cli.Context) {
 			!ctx.IsSet(utils.SepoliaFlag.Name) &&
 			!ctx.IsSet(utils.RinkebyFlag.Name) &&
 			!ctx.IsSet(utils.GoerliFlag.Name) &&
-			!ctx.IsSet(utils.KilnFlag.Name) &&
 			!ctx.IsSet(utils.DeveloperFlag.Name) {
 			// Nope, we're really on mainnet. Bump that cache up!
 			log.Info("Bumping default cache on mainnet", "provided", ctx.Int(utils.CacheFlag.Name), "updated", 4096)