Remove obsolete MSBuild tasks: DownloadFile, ZipArchive, UnzipArchive

[natemcmaster]

Delete these tasks:

• DownloadFile
• UnzipArchive
• ZipArchive

These are first-class citizens in MSBuild now thanks to @jeffkl.

cref dotnet/msbuild#3297
cref dotnet/msbuild#3291

[jeffkl]

Be advised that I added ZipDirectory which can only zip an entire directory. It looks like your ZipArchive accepts a set of files and a base directory so moving to ZipDirectory would require some modification to targets. If you need more complexity, perhaps you can contribute ZipArchive back to MSBuild.

[natemcmaster]

I'll double check for usage, but at the moment, these tasks aren't actually used AFAIK. They were an attempt to commonize tasks that existed in aspnet and dotnet build tools. I know aspnet has at least a few places where we zip a list of files instead of just a directory, but it may be possible to just make that use ZipDirectory instead.

[natemcmaster]

@chcosta PTAL. I didn't find anything using these tasks, so we're ok to remove them.

[chcosta]

Ack, I'll add this to my todo list (to take a look) tomorrow. Thanks!

[weshaggard]

I'm happy to see us move in this direction but are we using a new enough version of msbuild/sdk to get the built in ones? If not we might need to keep these until then.

[natemcmaster]

As far as I can tell, no one is using the Microsoft.DotNet.Build.Tasks.IO package at all. In fact, when I get dotnet/msbuild#3398 merged, I'm planning to remove that package entirely. Creating this package was a good proof of concept, but we've moved faster on getting stuff into MSBuild than we've moved on converging to a common set of build tools. These new tasks will be in MSBuild 15.8

[chcosta]

Sounds good to me, I don't have any further comments.