chore: remove build command (#846)

## Summary

`build` command has been deprecated for a long time, so I think it's
time to remove it.

## How was it tested?

Author: hezhizhen

internal/boxcli/build.go

@@ -43,7 +43,6 @@ func RootCmd() *cobra.Command {
 		SilenceUsage:  true,
 	}
 	command.AddCommand(AddCmd())
-	command.AddCommand(BuildCmd())
 	command.AddCommand(CloudCmd())
 	command.AddCommand(GenerateCmd())
 	command.AddCommand(globalCmd())