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.

cmd/tendermint/commands/run_node.go

@@ -70,10 +70,6 @@ func AddNodeFlags(cmd *cobra.Command) {
 
 	// rpc flags
 	cmd.Flags().String("rpc.laddr", config.RPC.ListenAddress, "RPC listen address. Port required")
-	cmd.Flags().String(
-		"rpc.grpc-laddr",
-		config.RPC.GRPCListenAddress,
-		"GRPC listen address (BroadcastTx only). Port required")
 	cmd.Flags().Bool("rpc.unsafe", config.RPC.Unsafe, "enabled unsafe rpc methods")
 	cmd.Flags().String("rpc.pprof-laddr", config.RPC.PprofListenAddress, "pprof listen address (https://golang.org/pkg/net/http/pprof)")