Fix a set of test issues caused by an old blob feed and try to fix up the nullable change we rushed in

[marcpopMSFT]

Fixes #45417

Changes included

• Disable nullable for netframework builds
• Fix the nullable logic for the stopwatch
• Remove DotnetCoreBlobFeed as this is a very old feed only needed for one test
• Delete the dotnet store test that needed that one feed

[marcpopMSFT]

@jaredpar I tried adding your suggestion of disabling nullable warnings for MT netfx builds but that doesn't work: #45420

I can confirm locally that DisableNullableWarnings is true in the netfix CLI.Utils build but I still get this warning: C:\repos\sdk\src\Cli\Microsoft.DotNet.Cli.Utils\Command.cs(82,21): error CS8602: Dereference of a possibly null reference. [C:\repos\sdk\src\Cli\Microsoft.DotNet.Cli.Utils\Microsoft.DotNet.Cli.Utils.csproj]

[v-wuzhai]

It could be due to a temporary issue with the network or storage account. Now, these tests have suddenly passed. Therefore, I will close this PR.

[marcpopMSFT]

Reactivating as this has templating test fixes and nullable fixes as well.

Additionally, I think we want the change to remove the blob. It has risk of keeping it around and removing simplifies things. Only one test failed which I'll talk to Daniel about.

Microsoft.NET.Publish.Tests.GivenThatWeWantToStoreAProjectWithDependencies.store_nativeonlyassets failed
error NU1101: Unable to find package runtime.osx.10.10-x64.Microsoft.NETCore.CoreDisTools. No packages exist with this id in source(s): dotnet-public, dotnet-tools, dotnet10, dotnet10-transport, dotnet6, dotnet7, dotnet8, dotnet8-transport, dotnet8-workloads, dotnet9, dotnet9-transport, Package source 1, Package source 2