In your TinyMCE configuration, just clear table_default_attributes, like this:

    [ModuleDependency(typeof(TinyMceInitialization))]
    public class TinyMceSettingsInitialization : IConfigurableModule
    {
        public void ConfigureContainer(ServiceConfigurationContext context)
        {
            context.Services.Configure<TinyMceConfiguration>(config =>
            {
                var tableSettings = new Dictionary<string, object>
                {
                    { "table_default_attributes", new {}}
                };

                config.Default().RawSettings(tableSettings);
            });
        }
    }

Yes! Very nice, thank you :)

Or, maybe better:

    [ModuleDependency(typeof(TinyMceInitialization))]
    public class TinyMceSettingsInitialization : IConfigurableModule
    {
        public void ConfigureContainer(ServiceConfigurationContext context)
        {
            context.Services.Configure<TinyMceConfiguration>(config =>
            {
                config.Default().AddSetting("table_default_attributes", new {});
            });
        }
    }

Yes, I added it using this syntax to to remove all default attributes and inline styles :)

                    .AddSetting("table_default_attributes", new { })
                    .AddSetting("table_default_styles", new { });