container issue: AddApplicationPart is not required in Asp.net core 3

From the looks of it, the call to AddApplicationPart is no longer required if the assembly has a reference to Asp.net core 3.

See commit: https://github.com/dotnet/aspnetcore/commit/200c9e21bdeaa6257d3f617b06e28f6d1bf42c69#diff-2e9e6b02c620405ad09d4cc327e452ca

And doc:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.applicationparts.applicationpartattribute?view=aspnetcore-3.1

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Add a nuget reference to Microsoft.Extensions.FileProviders.Embedded. #22643

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#### Document details

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* ID: 2784a724-e940-4c14-3b50-b063f78c2ee5
* Version Independent ID: 41175c3a-8cb5-856b-a7e0-931edb6394db
* Content: [Share controllers, views, Razor Pages and more with Application Parts in ASP.NET Core](https://docs.microsoft.com/en-gb/aspnet/core/mvc/advanced/app-parts?view=aspnetcore-3.1#feedback)
* Content Source: [aspnetcore/mvc/advanced/app-parts.md](https://github.com/aspnet/AspNetCore.Docs/blob/master/aspnetcore/mvc/advanced/app-parts.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-mvc**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**