[Snippets 5000] Delete source files where no project exists should pass

[BillWagner]

We continue to get false positives from the Snippets 5000 automation when we move old snippets to the new structure. Snippets5000 correctly ensures that the new location must have a build project, but, deleting the old files causes a false positive because source was removed and no project was built. See dotnet/docs#48299 for an example.

[adegeo]

Bill I don't know if this is valid. I left a single code file in those folders that it was triggered on. So, I deleted 9/10 of the code files, but because one remains, it correctly tried to compile the folder. If I had deleted all of the code files, I don't think it would have blocked me because it would have felt that the entire sample was deleted.