rpc: Remove the deprecated gRPC interface to the RPC service (#7121)

This change removes the partial gRPC interface to the RPC service, which was
deprecated in resolution of #6718.

Details:
- rpc: Remove the client and server interfaces and proto definitions.
- Remove the gRPC settings from the config library.
- Remove gRPC setup for the RPC service in the node startup.
- Fix various test helpers to remove gRPC bits.
- Remove the --rpc.grpc-laddr flag from the CLI.

Note that to satisfy the protobuf interface check, this change also includes a
temporary edit to buf.yaml, that I will revert after this is merged.

test/app/grpc_client.go

@@ -1,42 +0,0 @@
-package main
-
-import (
-	"encoding/hex"
-	"fmt"
-	"os"
-
-	"context"
-
-	tmjson "github.com/tendermint/tendermint/libs/json"
-	coregrpc "github.com/tendermint/tendermint/rpc/grpc"
-)
-
-var grpcAddr = "tcp://localhost:36656"
-
-func main() {
-	args := os.Args
-	if len(args) == 1 {
-		fmt.Println("Must enter a transaction to send (hex)")
-		os.Exit(1)
-	}
-	tx := args[1]
-	txBytes, err := hex.DecodeString(tx)
-	if err != nil {
-		fmt.Println("Invalid hex", err)
-		os.Exit(1)
-	}
-
-	clientGRPC := coregrpc.StartGRPCClient(grpcAddr)
-	res, err := clientGRPC.BroadcastTx(context.Background(), &coregrpc.RequestBroadcastTx{Tx: txBytes})
-	if err != nil {
-		fmt.Println(err)
-		os.Exit(1)
-	}
-
-	bz, err := tmjson.Marshal(res)
-	if err != nil {
-		fmt.Println(err)
-		os.Exit(1)
-	}
-	fmt.Println(string(bz))
-}