Remove duplicate common folders

[mairaw]

The common folder was copied to https://github.com/dotnet/docs/tree/master/samples/snippets/common and also to each individual language. I think we should only keep one copy of it.

[rpetrusha]

I agree.

[tompratt-AQ]

@mairaw Unless you have something more pressing for me to work on, I'd like to take on this issue. Since I've already deleted a few of these in other PRs, I familiar with the issue and how to proceed while making sure that the \common files are truly dupes.